Process for removing a target gas from a mixture of gases by swing adsorption

ABSTRACT

The present invention relates the separation of a target gas from a mixture of gases through the use of engineered structured adsorbent contactors in pressure swing adsorption and thermal swing adsorption processes. Preferably, the contactors contain engineered and substantially parallel flow channels wherein 20 volume percent or less of the open pore volume of the contactor, excluding the flow channels, is in the mesopore and macropore range.

This application claims the benefit of U.S. Provisional Application No.60/931,000 filed May 18, 2007.

FIELD OF THE INVENTION

The present invention relates to the separation of a target gas from amixture of gases using a swing adsorption process unit. The contactorsof the swing adsorption process unit are engineered structured adsorbentcontactors having a plurality of flow channels and wherein 20 volumepercent or less of the open pore volume of the contactors, excluding theflow channels, is in the meso and macropore range.

BACKGROUND OF THE INVENTION

Gas separation is important in various industries and can typically beaccomplished by flowing a mixture of gases over an adsorbent thatpreferentially adsorbs a more readily adsorbed component relative to aless readily adsorbed component of the mixture. One of the moreimportant gas separation techniques is pressure swing adsorption (PSA).PSA processes rely on the fact that under pressure gases tend to beadsorbed within the pore structure of the microporous adsorbentmaterials or within the free volume of a polymeric material. The higherthe pressure, the more gas is adsorbed. When the pressure is reduced,the gas is released, or desorbed. PSA processes can be used to separategases in a mixture because different gases tend to fill the micropore orfree volume of the adsorbent to different extents. If a gas mixture,such as natural gas, for example, is passed under pressure through avessel containing polymeric or microporous adsorbent that fills withmore nitrogen than it does methane, part or all of the nitrogen willstay in the sorbent bed, and the gas coming out of the vessel will beenriched in methane. When the bed reaches the end of its capacity toadsorb nitrogen, it can be regenerated by reducing the pressure, therebyreleasing the adsorbed nitrogen. It is then ready for another cycle.

Another important gas separation technique is temperature swingadsorption (TSA). TSA processes also rely on the fact that underpressure gases tend to be adsorbed within the pore structure of themicroporous adsorbent materials or within the free volume of a polymericmaterial. When the temperature of the adsorbent is increased, the gas isreleased, or desorbed. By cyclically swinging the temperature ofadsorbent beds, TSA processes can be used to separate gases in a mixturewhen used with an adsorbent that selectively picks up one or more of thecomponents in the gas mixture.

Adsorbents for PSA systems are usually very porous materials chosenbecause of their large surface area. Typical adsorbents are activatedcarbons, silica gels, aluminas and zeolites. In some cases a polymericmaterial can be used as the adsorbent material. Though the gas adsorbedon the interior surfaces of microporous materials may consist of a layeronly one, or at most a few molecules thick, surface areas of severalhundred square meters per gram enable the adsorption of a significantportion of the adsorbent's weight in gas.

Different molecules can have different affinities for adsorption intothe pore structure or open volume of the adsorbent. This provides onemechanism for the adsorbent to discriminate between different gasses. Inaddition to their affinity for different gases, zeolites and some typesof activated carbons, called carbon molecular sieves, may utilize theirmolecular sieve characteristics to exclude or slow the diffusion of somegas molecules into their structure. This provides a mechanism forselective adsorption based on the size of the molecules and usuallyrestricts the ability of the larger molecules to be adsorbed. Either ofthese mechanisms can be employed to selectively fill the microporestructure of an adsorbent with one or more species from amulti-component gas mixture. The molecular species that selectively fillthe micropores or open volume of the adsorbent are usually referred toas the “heavy” components and the molecular species that do notselectively fill the micropores or open volume of the adsorbent areusually referred to as the “light” components.

An early teaching of a PSA process having a multi-bed system is found inU.S. Pat. No. 3,430,418 wherein a system having at least four beds isdescribed. This '418 patent describes a cyclic PSA processing sequencethat includes in each bed: (1) higher pressure adsorption with releaseof product effluent from the product end of the bed; (2) co-currentdepressurization to intermediate pressure with release of void space gasfrom the product end thereof; (3) countercurrent depressurization to alower pressure; (4) purge; and (5) repressurization. The void space gasreleased during the co-current depressurization step is commonlyemployed for pressure equalization purposes and to provide purge gas toa bed at its lower desorption pressure. Another conventional PSAprocesses using three sorbent beds is disclosed in U.S. Pat. No.3,738,087. Conventional PSA processes are typically able to recover onlyone of the key components (i.e., light or heavy) at high purity and areunable to make a complete separation and separate both components with ahigh recovery. The light component usually has a low recovery factor.Recovery of the light component usually drops even lower when the feedgas is introduced at higher pressures (i.e., pressures above 500 psig).

For the recovery of a purified strongly adsorbed “heavy” component, anadditional step is usually necessary, namely, rinsing of the bed with aheavy component to displace the light component from the bed prior todepressurization. The rinsing step is well known in the art. Theproblems associated with these processes are the following: (a) if therinsing is complete and the light component is completely displaced fromthe bed, substantially pure heavy component can be obtained, but theadsorption front of the heavy component breaks through to the lightcomponent and the latter cannot be recovered at high purity; (b) if thedisplacement of the light component is incomplete, the typicalconcentration profile of the heavy component in the bed is not optimumand as such the bed is depressurized countercurrently to recover theheavy key component at the feed end, the light component still presentin the bed reaches the feed end very rapidly and the purity of the heavycomponent drops. Therefore it is not practical in the prior art toobtain both key components at high purity in a single PSA unit.

The faster the beds perform steps to complete a cycle, the smaller thebeds can be when used to process a given hourly feed gas flow. Severalother approaches to reducing cycle time in PSA processes have emergedwhich use rotary valve technologies as disclosed in U.S. Pat. Nos.4,801,308; 4,816,121; 4,968,329; 5,082,473; 5,256,172; 6,051,050;6,056,80; 6,063,161; 6,406,523; 6,629,525; 6,651,658 and 6,691,702. Aparallel channel (or parallel passage) contactor with a structuredadsorbent is used to allow for efficient mass transfer in these rapidcycle pressure swing adsorption processes. Approaches to constructingparallel passage contactors with structured adsorbents have beendisclosed in US20060169142 A1, US20060048648 A1, WO2006074343 A2,WO2006017940 A1, WO2005070518 A1, and WO2005032694 A1.

In a parallel channel contactor, the adsorbent lines the wall of theflow channel which can be formed from the space between parallel platesor the open path through a duct or tube. When parallel plates are usedto form the parallel channel, a spacer may be present in the space ofthe parallel channel. An example of a spacer-less parallel passagecontactor as provided in US20040197596 A1 and an example of a parallelpassage contactor with a high density adsorbent structure is given inUS20050129952A1. In all cases, the adsorbent used to line the parallelchannel contains both mesopores and macropores.

Mesopores and macropores are known in the art to improve the masstransfer characteristics of adsorbents used in either a parallel channelcontactor or conventional packed bed contactors. Improvements in masstransfer characteristics from the presence of mesopores and macroporesin conventional packed bed contactors have been widely discussed. Seefor example U.S. Pat. Nos. 6,436,171 and 6,284,021. Improvements in masstransfer characteristics from the presence of mesopores and macroporesin parallel channel contactors are discussed in EP1413348 A1. As such,the prior art teaches that a large number of mesopores and macroporesare needed in an adsorbent particle or layer of adsorbent in order tohave mass transfer characteristics good enough to operate a pressureswing adsorption cycle. The inventors hereof have unexpected found thatadequate mass transfer characteristics can be attained without asignificant amount of mesopores and/or macropores providing easy accessto the micropore structure in the adsorbent where selective separationoccurs.

While there are various teachings in the art with respect to newadsorbent materials, new and improved parallel channel contactors, andimproved rapid cycle PSA equipment, none of these to date present aviable solution to the problem of producing good recovery of the lightcomponent and purity when the feed gas is at very high-pressure. This isa critical issue since natural gas is often produced at high pressures(500-7000 psi) and methane acts as a light component in the adsorptionprocess. Many gas fields also contain significant levels of H₂O, H₂S,CO₂, N₂, mercaptans and/or heavy hydrocarbons that have to be removed tovarious degrees before the gas can be transported to market. It ispreferred that as much of the acid gases H₂S and CO₂ be removed fromnatural gas as possible. In all natural gas separations, methane is avaluable component and acts as a light component in swing adsorptionprocesses. Small increases in recovery of this light component canresult in significant improvements in process economics and also serveto prevent unwanted resource loss. It is desirable to recover more than80 vol. %, preferably more than 90 vol. % of the methane whendetrimental impurities are removed. While various processes exist forremoving CO₂, H₂S, and N₂ from natural gas there remains a need forprocesses and materials that will perform this recovery moreefficiently, at lower costs, and at higher hydrocarbon yields,particularly at higher methane yields.

Similarly, for other gaseous feed streams, the prior art describesseveral ways to recover high amounts of the heavy components in a heavycomponent rich “reject” stream, but cannot achieve as high a recovery ofthe light components in the light component rich product stream. Thisdifference in recoveries becomes greater as the feed pressure increases.

SUMMARY OF THE INVENTION

In one embodiment of the present invention there is provided a processfor removing a target gas component from a gas mixture containing saidtarget gas component and a second gas component, which processcomprises:

a) conducting said gas mixture to a swing adsorption gas separation unitwherein the gas separation unit contains at least one adsorbentcontactor comprising a gas inlet and a gas outlet, wherein the gas inletand the gas outlet are in fluid connection by a plurality of open flowchannels wherein the surfaces of the open flow channels are comprised ofan adsorbent material that has a selectivity for said target gascomponent over said second gas component greater than 5, wherein thecontactor has less than about 20% of its open pore volume in pores withdiameters greater than about 20 angstroms and less than about 1 micron,and wherein at least a portion of said target gas component is adsorbedinto said adsorbent material, thereby resulting in a product streamdepleted of said target gas component;

b) collecting said the product stream;

c) desorbing the adsorbed gases from said adsorbent material, therebyresulting in a waste gas stream rich in said target gas component; and

d) collecting said waste gas stream.

In a preferred embodiment, the adsorbent material is comprised of astructured adsorbent material selected from the group consisting ofzeolites, titanosilicates, ferrosilicates, stannosilicates,aluminophosphate molecular sieves (AlPOs), and silicoaluminophosphatemolecular sieves (SAPOs) and carbon molecular sieves.

In another preferred embodiment, the adsorbent material is comprised ofan 8-ring zeolite that has a Si to Al ratio of about 1:1 to about1000:1. In a further preferred embodiment, the 8-ring zeolite is DDR. Inyet another further preferred embodiment, the 8-ring zeolite is selectedfrom Sigma-1 and ZSM-58.

In a preferred embodiment, the adsorbent contactor is a parallel channelcontactor comprising structured (engineered) adsorbents in whichsubstantially parallel flow channels are incorporated into the adsorbentstructure.

In another embodiment, the channel gap of the open flow channels in theparallel contactor is from about 5 to about 1000 microns.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 hereof is a representation of one embodiment of a parallelchannel contactor of the present invention in the form of a monolithdirectly formed from the microporous adsorbent of the present inventionand containing a plurality of parallel channels.

FIG. 2 hereof is a cross-sectional representation along the longitudinalaxis of the monolith of FIG. 1.

FIG. 3 hereof is a representation of a magnified section of thecross-sectional view of the monolith of FIG. 2 showing the detailedstructure of the adsorbent layer along with a blocking agent occupyingsome of the meso and macropores.

FIG. 4 hereof is another representation of an embodiment of a parallelchannel contactor of the present invention in the form of a coatedmonolith where the adsorbent layer is coated onto the channel wall.

FIG. 5 hereof is an electron micrograph of the surface of a DDR filmthat is suitable to act as the adsorbent layer of the contactors of thepresent invention with a volume fraction of open meso and microporesthat is less than about 7%.

FIG. 6 is an electron micrograph of the surface of an MFI film that isalso suitable to act as an adsorbent layer of the contactors of thepresent invention with a volume fraction of open meso and microporesthat is less than about 7%.

FIG. 7 hereof represents another embodiment of the present invention inwhich the parallel channel contactor is in the form of a coated monolithfor TSA applications where the adsorbent layer is coated onto thechannel walls of a preformed monolith.

FIG. 8 hereof is a representation of a parallel channel contactor of thepresent invention in the form of an array of hollow fibers.

FIG. 9 hereof is yet another representation of a parallel channelcontactor of the present invention but in the form of a hollow fibercontactor for TSA applications.

FIG. 10 hereof is another representation of a hollow fiber contactor forTSA as shown in FIG. 9 but with the outer surfaces of the housing forthe contactor rendered transparent. Dotted lines are used to indicatethe edges of the outer surface.

FIG. 11 hereof is a representation of an embodiment of the presentinvention wherein the parallel contactor is of the laminate type.

FIG. 12 hereof is a schematic diagram of a preferred five stepsPSA/RCPSA process for treating a stream containing about 20 vol. % CO₂and about 80 vol. % CH₄.

FIG. 13 hereof is a schematic diagram of a preferred five stepsPSA/RCPSA process for treating a stream containing about 2 vol. % N₂ andabout 98vol. % CH₄.

FIG. 14 hereof is a schematic diagram of an integrated process utilizinga turboexpander and a PSA process of the present invention.

FIG. 15 hereof is a schematic representation of a preferred procedurefor measuring the volume fraction of mesopores and macropores ofadsorbent contactors of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to adsorbent contactors for use inswing adsorption processes, which adsorbent contactors contain aplurality of flow channels and which contactors contain 20 vol. % orless, preferably 15 vol. % or less, more preferably 10 vol. % or less,and most preferably 5 vol. % or less of their open pore volume in poresin the mesopore and macropore size range. The term “adsorbent contactor”as utilized herein includes both structured and unstructured adsorbentcontactors. The preferred contactors of the present invention are a typeof structured adsorbent contactor entitled herein as “parallel channelcontactors” for use in thermal swing adsorption (TSA) and various typesof pressure swing adsorption processes including conventional pressureswing adsorption (PSA), and partial pressure swing or displacement purgeadsorption (PPSA) technologies. These swing adsorption processes can beconducted with rapid cycles, in which case they are referred to as rapidcycle thermal swing adsorption (RCTSA), rapid cycle pressure swingadsorption (RCPSA), and rapid cycle partial pressure swing ordisplacement purge adsorption (RCPPSA) technologies. The term swingadsorption processes shall be taken to include all of these processes(i.e. TSA, PSA, PPSA, RCTSA, RCPSA, and RCPPSA) including combinationsof these processes. Such processes require efficient contact of a gasmixture with a solid adsorbent material. It should also be noted thatunless otherwise noted herein or by reference to specific “geometricshapes” (in which case would apply only to structured adsorbentcontactors), that all preferred embodiments as described in thisapplication, such as, but limited to, contactor voidages, separationcomponents and efficiencies, operating conditions, preferred materials,etc., apply to both structured and unstructured adsorbent contactors ofthe present invention as described herein.

The structure of parallel channel contactors, including fixed surfaceson which the adsorbent or other active material is held, providessignificant benefits over previous conventional gas separation methods,such as vessels containing adsorbent beads or extruded adsorbentparticles. “Parallel channel contactors” are defined herein as a subsetof adsorbent contactors comprising structured (engineered) adsorbents inwhich substantially parallel flow channels are incorporated into theadsorbent structure. These flow channels may be formed by a variety ofmeans, many of which are described herein and in addition to theadsorbent material, the adsorbent structure may contain items such as,but not limited to, support materials, heat sink materials, voidreduction components, etc., which are described more fully herein.

Swing adsorption processes are all well known to those having ordinaryskill in the art and they can be applied to remove a variety of targetgases from a wide variety of gas mixtures. It is possible tosignificantly improve the recovery percentage of the light component ofa gas mixture by use of the present invention. The “light component” asutilized herein is taken to be the species, or molecular component, orcomponents that are not preferentially taken up by the adsorbent in theadsorption step of the process. Conversely, the “heavy component” asutilized herein is taken to be the species, or molecular component, orcomponents that are preferentially taken up by the adsorbent in theadsorption step of the process. With the contactors of the presentinvention, it has been unexpectedly discovered that total recovery ofthe light component achieved in the swing adsorption process can begreater than about 80 vol. %, more preferably greater than about 85 vol.%, even more preferably greater than about 90 vol. %, and mostpreferably greater than about 95 vol. % of the content of the lightcomponent introduced into the process. Recovery of the light componentis defined as the time averaged molar flow rate of the light componentin the product stream divided by the time averaged molar flow rate ofthe light component in the feedstream. Similarly, recovery of the heavycomponent is defined as the time averaged molar flow rate of the heavycomponent in the product stream divided by the time averaged molar flowrate of the heavy component in the feedstream.

The structured adsorbent contactors of the present invention contain avery low volume fraction of open mesopores and macropores. That is, thestructured bed adsorbent contactors of the present invention containless than about 20 vol. %, preferably less than about 15 vol. %, morepreferably less than about 10 vol. %, and most preferably less thanabout 5 vol. % of their pore volume in open pores in the mesopore andmacropore size range. Mesopores are defined by the IUPAC to be poreswith sizes in the 20 to 500 angstrom size range. Macropores are definedherein to be pores with sizes greater than 500 angstroms and less than 1micron. Because the flow channels are larger than 1 micron in size, theyare not considered to be part of the macropore volume. By open pores wemean mesopores and macropores that are not occupied by a blocking agentand that are capable of being occupied, essentially non-selectively, bycomponents of a gas mixture. Different test methods as described beloware to be used to measure the volume fraction of open pores in acontactor depending on the structure of the contactor.

Open pore volume (in percent or volume percent) is defined herein as thevolume of the pores in the adsorbent that are between 20 angstroms and10,000 angstroms (1 micron) in diameter divided by the total volume ofthe contactor that is occupied by the adsorbent material includingassociated mesopores and macropores in the adsorbent structure. “Sweptvolumes” such as engineering flow channels as well as the volumeoccupied by any non-adsorbent material, such as but not limited to,support materials, blocking agents, thermal masses, etc., are notincluded in the amount of volume occupied by the adsorbent material.

The preferred test for determining the volume fraction of open mesoporesand macropores of the contactor is defined as follows and involves ananalysis of the isotherm of a condensable vapor adsorbed by thecontactor. A liquid which has a vapor pressure greater than 0.1 torr atthe temperature of the test is a suitable material that can be used toproduce a condensable vapor. At 20° C., water, hexane, trimehtlybenzene,toluene, xylenes, and isooctane have sufficiently high vapor pressuresthat they can be used as condensable vapors. In the adsorption branch ofthe isotherm (obtained by increasing the pressure of the condensablevapor), capillary condensation fills empty micropore, mesopore, and muchof the empty macropore volume with liquid. In the desorption branchmicropores, mesopores, and macropores pores filled with liquid areemptied. It is well known that there is a hysteresis between theadsorption and desorption branches of the isotherm. Detailed analysis ofthe adsorption isotherm relies in part on the Kelvin equation which iswell known to those skilled in the art. The detailed analysis provides ameasurement of the volume fraction of the mesopores and macropores inthe structured adsorbent. The preferred measurement technique describedin the following paragraphs derives from these principles.

If a liquid blocking agent is not used in the contactor the procedureoutlined in this paragraph is used to measure the volume of openmesopores and macropores of the subject contactor. This measurement isperformed on either the entire contactor or a representative portion ofthe contactor. A representative portion of the contactor contains aleast an entire cross section of the contactor and has a mass that isbetween 10% and 100% of the mass of the contactor. Additionally, themass of the contactor used in this measurement should be more than 50grams. A preferred procedure is represented in FIG. 15 hereof whereinthe initial steps of the procedure involve placing the contactor 1101 ora representative portion of the contactor, into a sealable vacuum tightcontainer 1103 and evacuating the container. Chamber 1103 is designed sothat the volume defined by the exterior surface of the contactor 1101 isat least 50% of the interior volume of the chamber. The vacuum tightcontainer is equipped with a pressure transducer 1173 and a cable 1171to power the transducer and transmit the signal generated. Thetransducer is chosen so that it can measure pressures with an accuracyof 0.01 torr. A suitable transducer for such a measurement is acapacitance manometer.

The vacuum tight vessel 1103 and all of the equipment used to evacuatethe vessel and fill the vessel with known quantities of the condensablevapor are located in an isothermal chamber 1191. The isothermal chamber1191 is kept at a temperature of 30.00 (+/−0.01)° C. To make sure thatthe vacuum tight container 1103 and vessels 1125 and 1141 are at aconstant temperature before and after each step used to dose hexane intothe contactor their temperature is monitored with thermocouples 1195,1197 and 1199 that can be read with a resolution of 0.01° C. Thethermocouple readings may differ by a small amount because the accuracyof thermocouple readings is generally 0.2° C. The important issue isthat before and after each hexane dosing step the readings ofthermocouples 1195, 1197 and 1199 remain constant to within +/−0.01° C.Initially all valves (1113, 1127, 1149, 1151, 1153 and 1165) in thesystem are closed. It is preferred that the valves be air actuated(instead of solenoid activated) so that do not heat the gas or pipingwhen they are opened. Before the measurement begins the equipment usedto fill hexane vapor (chosen as an example of a condensable vapor at 30°C.) into vessel 1103 must be leak checked. This is done by openingvalves 1127, 1151, and 1153, allowing line 1119 that is attached to avacuum pump to evacuate vessels 1125 and 1141.

Pressure in these vessels is measured with transducers 1163 and 1143which have cables 1161 and 1145, respectively to power them and transmitthe signals generated. The transducer is chosen so that they can measurepressures with an accuracy of 0.01 torr and suitable transducers arecapacitance manometers. The vessel 1125 and 1141 are evacuated so thattransducers 1163 and 1143 read a pressure of less than 0.03 torr. Valve1127 is then closed and the system is considered leak tight if thepressure recorded by transducers 1163 and 1143 does not rise by morethan 0.02 torr over a three hour period. At this point valves 1153 and1151 are closed and the evacuated vessel 1141 is filled with hexanethrough line 1147 by opening valve 1149. Line 1147 was originally filledwith liquid hexane. To stop the filling of reservoir 1141, valve 1149 isshut.

A procedure is then instituted to remove any impurity gases that mayhave been carried into vessel 1141 during the hexane filling step. Gasesare removed by opening valve 1127 and then opening valve 1151 for aperiod of 2 minutes dropping the pressure in vessel 1141 thus allowingthe hexane to boil. This degassing procedure is repeated five timesbefore the hexane in vessel 1141 can be used. At this point valve 1153is closed and valve 1127 is open, thus evacuating line 1129. Valve 1153is then opened pulling a vacuum on vessel 1125. Adsorbed molecules inthe contactor or a representative piece of the contactor 1101 in chamber1103 are then removed by opening valve 1113. This allows line 1115 thatis attached to a vacuum pump to evacuate chamber 1103. The evacuationprocedure continues until the pressure in the vessel 1103 falls below0.03 torr. At this point valve 1113 is closed and the pressure in thechamber is monitored for an hour. If the pressure in the chamber risesby more than 0.02 torr during this time period then the evacuationprocedure is repeated. If the evacuation procedure has to be repeatedmore than ten times then alternative means of removing molecules fromthe contactor 1101 should be employed (such as heating the contactor).After adsorbed molecules have been successfully removed from thecontactor, valves 1127 and 1153 are closed.

Valve 1151 is then opened allowing line 1129 to fill with hexane vapor.Under the conditions of the test this will be approximately 187 torr ofhexane vapor. Valve 1151 is then closed and valve 1153 is openedallowing the vapor in the line to expand into vessel 1125. Because ofthe difference in volume between line 1129 and vessel 1125 the pressuremeasured by transducer 1163 will be less than 5 torr. Valve 1153 is thenclosed and this filling procedure is repeated until the pressure invessel 1125 is approximately 5 torr. At this point valve 1165 is openedto dose the contactor 1101 with hexane vapor. The pressure in vessel1125 drops because of gas expansion into chamber 1103 and possibleadsorption of molecules of hexane into micropores of the contactor.After the pressure in vessel 1125 stops dropping and stabilizes, thepressure is recorded and the number of moles of hexane transferred intochamber 1103 is computed from the ideal gas law. This requires knowledge(previous measurement) of the interior volume of vessel 1125 and itsassociated piping.

The moles of gas transferred into vessel 1103 that would be needed tofill the gas space inside it are computed using the ideal gas law withthe pressure measured by transducer 1173. Again, knowledge of theavailable gas space in chamber 1103 is required for the calculation. Theinterior volume of chamber 1103 is known (previous measurement) and theavailable gas volume is computed by subtracting from the interior volumeof vessel 1103 the exterior volume of the contactor and adding back thevolume of the flow channels in the contactor. Errors in knowledge of theexterior volume of the contactor or the volume of the flow channels willnot significantly affect the measurement of the total open mesopore andmacropore volumes. The quantity which is the difference between theexterior volume of the contactor and the volume of the flow channelsonly has to be known to an accuracy of 20%.

The moles of gas adsorbed by the contactor is then the differencebetween the moles of gas transferred and the moles of gas required tofill the available gas space in vessel 1103 and its associated piping.After completing this dosing step and the evaluation of the moles of gasadsorbed in the contactor 1101 valve 1165 is closed and pressure invessel 1125 is increased 5 more torr by repeating the procedureoriginally used to fill it with hexane vapor. The dosing step to adsorbmore molecules into the contactor is then repeated. The filling anddosing steps continue to be repeated until the pressure in vessel 1125at the end of a dosing step is within +/−2.5 torr of 15% of the readingof pressure transducer 1143. This range is expected to be between 25.5and 30.5 torr. From this point onward in the procedure the additionalmoles of hexane that are adsorbed in the contactor are considered tofill the mesopores and macropores. The filling and dosing steps arecontinued until the pressure in chamber 1103 housing the contactorexceeds 95% of the pressure read by transducer 1143.

Filling and dosing steps are continued in a manner such that thepressure in vessel 1125 is only increased by 1 torr in each fillingstep. When the pressure read by transducer 1173 at the end of a dosingstep exceeds 98.5% of the pressure read by transducer 1143 the pressureincrease in vessel 1125 during a filling step is decreased to 0.5 torr.When the pressure read by transducer 1173 at the end of a dosing stepexceeds 99.25% of the pressure read by transducer 1143 the pressureincrease in vessel 1125 during a filling step is decreased to 0.05 torr.The filling and dosing steps are then continued until the pressure readby the transducer 1173 at the end of a dosing step exceeds 99.6% of thepressure read by transducer 1143. The pressure at which the experimentis ended is expected to be approximately 186.4 torr. The total number ofmoles adsorbed by the contactor from the point at which at the end of adosing step transducer 1161 was in a range between within +/−2.5 torr of15% of the reading of pressure transducer 1143 and the point at whichthe pressure read by the transducer 1173 at the end of a dosing stepexceeded 99.6% of the pressure read by transducer 1143 is the totalnumber of moles of hexane adsorbed in the mesopore and macropore volumeof the contactor. The volume of the open mesopores and macropores in thecontactor is then determined by multiplying this number of moles by themolar volume of hexane. The open pore volume as expressed in a volumefraction as used herein is then obtained by dividing the volume of openmesopores and macropores determined by this test by the total volume ofthe contactor that is occupied by the adsorbent material as definedprior. Although the open pore volume for the contactor is determined bythe test procedure described above, scanning electron microscopy may beused to further confirm the relative volume of mesopores and macroporesin the sample. When scanning electron microscopy is used the surface aswell as a cross section of the contactor should be imaged.

If a liquid material is used as a blocking agent in the formulation ofthe contactor, and the contactor is operated under conditions where thepores remain substantially fully filled with liquid no assay asdescribed above is required to determine the open pore volume of thecontactor. In this contactor configuration, the mesopores and macroporesof the contactor will remain filled with liquid as long as the liquidremains condensable at the operating temperature of the contactor andthe feed flowing into the contactor is fully saturated with the vapor ofthe liquid at the inlet temperature and pressure. In this case, there isno open mesopore or macropore volume in the contactor because it has allbeen filled-in by the condensable vapor. If under operating conditions(i.e., inlet temperature and pressure) the feed flowing into thecontactor is only partially saturated with the vapor of the liquid thensome fraction of the mesopore or macropore volume will remain open. Thedegree of saturation is characterized by a liquid activity (a_(liquid))that is the ratio of the partial pressure of the vapor of the liquid inthe flowing gas stream to the saturated vapor pressure of the liquid atthe temperature of the contactor (i.e. P_(i)/P_(sat)). The amount ofopen mesopore and macropore volume increases as the partial pressure ofthe condensable liquid in the feed decreases. To determine the volume ofmesopores and macropores under operating conditions the liquid materialused in the formulation of the contactor is removed from the mesoporesand macropores of the contactor by drying. Liquid can be dried-out ofthe mesopores and macropores of the contactor by heating it in a sealedcontainer while drawing a vacuum or by heating it while passing asubstantially pure purge gas, such as He, over the contactor. Onceliquid has been removed from the mesopores and macropores of thecontactor, the assay method previously described can be conducted. Inthis assay the amount of gas adsorbed by the contactor is plottedagainst the hexane activity (a_(hexane)) which is the hexane pressure atthe end of an adsorption step divided by the saturated hexane vaporpressure. The amount of open mesopores and macropores that would beexpected in operation is then determined from the cumulative number ofmoles of hexane adsorbed between the point at which the hexane activityexceeds a_(liquid) and the point at which the experiment is terminated(i.e., when the pressure read by the transducer 1173 at the end of adosing step exceeds 99.6% of the pressure read by transducer 1143).Again the molar volume of hexane is used to compute the actual openmesopore and macropore volume. The open pore volume as expressed in avolume fraction as used herein is then obtained by dividing the volumeof open mesopores and macropores determined by this test by the totalvolume of the contactor that is occupied by the adsorbent material asdefined prior.

In equilibrium controlled swing adsorption processes most of theselectivity is imparted by the equilibrium adsorption properties of theadsorbent, and the competitive adsorption isotherm of the light productin the micropores or free volume of the adsorbent is not favored. In akinetically controlled swing adsorption processes most of theselectivity is imparted by the diffusional properties of the adsorbentand the transport diffusion coefficient in the micropores and freevolume of the adsorbent of the light species is less than that of theheavier species. Also, in kinetically controlled swing adsorptionprocesses with microporous adsorbents the diffusional selectivity canarise from diffusion differences in the micropores of the adsorbent orfrom a selective diffusional surface resistance in the crystals orparticles that make-up the adsorbent.

It will be understood that the term PSA, unless preceded by the term“conventional” or “rapid cycle” refers collectively to all pressureswing adsorption processes including conventional PSA, RCPSA and PPSA.In PSA processes, a gaseous mixture is conducted under pressure for aperiod of time over a first bed of a solid sorbent that is selective, orrelatively selective, for one or more components, usually regarded as acontaminant, that is to be removed from the gaseous mixture. Thecomponents that are selectively adsorbed are referred to as the heavycomponent and the weakly adsorbed components that pass through the bedare referred to as the light components. It is possible to remove two ormore contaminants simultaneously but for convenience, the component orcomponents, that are to be removed by selective adsorption will bereferred to in the singular and referred to as a contaminant or heavycomponent.

Unless otherwise noted, the term “selectivity” as used herein is basedon binary (pairwise) comparison of the molar concentration of componentsin the feed stream and the total number of moles of these componentsadsorbed by the particular adsorbent during the adsorption step of theprocess cycle under the specific system operating conditions andfeedstream composition. For a feed containing component A, component B,as well as additional components, an adsorbent that has a greater“selectivity” for component A than component B will have at the end ofthe adsorption step of the swing adsorption process cycle a ratio:U _(A)=(total moles of A in the adsorbent)/(molar concentration of A inthe feed)that is greater than the ratio:U _(B)=(total moles of B in the adsorbent)/(molar concentration of B inthe feed)Where U_(A) is the “Adsorption Uptake of component A” and U_(B) is the“Adsorption Uptake of component B”.Therefore for an adsorbent having a selectivity for component A overcomponent B that is greater than one:Selectivity=U _(A) /U _(B)(where U _(A) >U _(B)).Amongst a comparison of different components in the feed, the componentwith the smallest ratio of the total moles picked up in the adsorbent toits molar concentration in the feed is the lightest component in theswing adsorption process. This means that the molar concentration of thelightest component in the stream coming out during the adsorption stepis greater than the molar concentration of that lightest component inthe feed. The adsorbent contactors of the present invention have aselectivity for a first component (e.g., component A) over a secondcomponent (e.g., component B) of at least 5, more preferably aselectivity for a first component over a second component of at least10, and most preferably a selectivity for a first component over asecond component of at least 25.

Examples of components are molecules such as molecular nitrogen, N₂, orcompounds, such as carbon dioxide, CO₂, and methane, CH₄. In a preferredembodiment of the present invention, the adsorbent contactor has aselectivity for CO₂ over CH₄ of at least 5, more preferably aselectivity for CO₂ over CH₄ of at least 10, and most preferably aselectivity for CO₂ over CH₄ of at least 25. In another preferredembodiment of the present invention, the adsorbent contactor has aselectivity for N₂ over CH₄ of at least 5, more preferably a selectivityfor N₂ over CH₄ of at least 10, and most preferably a selectivity for N₂over CH₄ of at least 25. In yet another preferred embodiment of thepresent invention, the adsorbent contactor has a selectivity for H₂Sover CH₄ of at least 5, more preferably a selectivity for H₂S over CH₄of at least 10, and most preferably a selectivity for H₂S over CH₄ of atleast 25.

In a preferred embodiment of the present invention, the adsorbent has a“kinetic selectivity” for two or more gas components. As used herein,the term “kinetic selectivity” is defined as the ratio of singlecomponent diffusion coefficients, D (in m²/sec), for two differentspecies. These single component diffusion coefficients are also known asthe Stefan-Maxwell transport diffusion coefficients that are measuredfor a given adsorbent for a given pure gas component. Therefore, forexample, the kinetic selectivity for a particular adsorbent forcomponent A with respect to component B would be equal to D_(A)/D_(B).The single component diffusion coefficients for a material can bedetermined by tests well known in the adsorptive materials art. Thepreferred way to measure the kinetic diffusion coefficient is with afrequency response technique described by Reyes et al. in “FrequencyModulation Methods for Diffusion and Adsorption Measurements in PorousSolids”, J. Phys. Chem. B. 101, pages 614-622, 1997. In a kineticallycontrolled separation it is preferred that kinetic selectivity (i.e.,D_(A)/D_(B)) of the selected adsorbent for the first component (e.g.,Component A) with respect to the second component (e.g., Component B) begreater than 5, more preferably greater than 20, even more preferablygreater than 50.

In another preferred embodiment of the present invention, the adsorbenthas an “equilibrium selectivity” for two or more gas components. As usedherein, the term “equilibrium selectivity” is defined in terms of theslope of the single component uptake into the adsorbent (in μmole/g) vs.pressure (in torr) in the linear portion, or “Henry's regime”, of theuptake isotherm for a given adsorbent for a given pure component. Theslope of this line is called herein the Henrys constant or “equilibriumuptake slope”, or “H”. The “equilibrium selectivity” is defined in termsof a binary (or pairwise) comparison of the Henrys constants ofdifferent components in the feed for a particular adsorbent. Therefore,for example, the equilibrium selectivity for a particular adsorbent forcomponent A with respect to component B would be H_(A)/H_(B). It ispreferred that in an equilibrium controlled separation the equilibriumselectivity (i.e., H_(A)/H_(B)) of the selected adsorbent for the firstcomponent (e.g., Component A) with respect to the second component(e.g., Component B) be greater than 5, more preferably greater than 20,even more preferably greater than 50.

In the PSA process, the gaseous mixture is passed over a firstadsorption bed in a first vessel and a light component enriched productstream emerges from the bed depleted in the contaminant, or heavycomponent, which remains sorbed in the bed. After a predetermined timeor, alternatively when a break-through of the contaminant or heavycomponent is observed, the flow of the gaseous mixture is switched to asecond adsorption bed in a second vessel for the purification tocontinue. While the second bed is in adsorption service, the sorbedcontaminant, or heavy component is removed from the first adsorption bedby a reduction in pressure. In some embodiments, the reduction inpressure is accompanied by a reverse flow of gas to assist in desorbingthe heavy component. As the pressure in the vessels is reduced, theheavy component previously adsorbed in the bed is progressively desorbedto a heavy component enriched product stream. When desorption iscomplete, the sorbent bed may be purged with an inert gas stream, e.g.,nitrogen or a purified stream of process gas. Purging may also befacilitated by the use of a higher temperature purge gas stream.

After the first bed has been regenerated so that it is again ready foradsorption service, the flow of the gaseous mixture is switched from thesecond bed to the first bed, and the second bed is regenerated. Thetotal cycle time is the length of time from when the gaseous mixture isfirst conducted to the first bed in a first cycle to the time when thegaseous mixture is first conducted to the first bed in the immediatelysucceeding cycle, i.e., after a single regeneration of the first bed.The use of third, fourth, fifth, etc. vessels in addition to the secondvessel can serve to increase cycle time when the adsorption cycle timefor the bed is shorter than the cycle times for the desorption & purgingcycles for the bed.

Conventional PSA processes suffer from several inherent disadvantages.For example, conventional PSA units are typically more costly to buildand operate and are significantly larger in size for the same amount oftarget gas that needs to be recovered from a target-gas containing gasstream, such as natural gas, as compared to RCPSA. Also, a conventionalPSA unit will generally have cycle times in excess of one minute,typically in excess of 2 to 4 minutes due to time limitations requiredto allow diffusion of the components through the larger beds utilized inconventional PSA and the equipment configuration involved. In contrast,RCPSA generally has a total cycle times of less than one minute. Thetotal cycle times of RCPSA may be less than 30 seconds, preferably lessthan 15 seconds, more preferably less than 10 seconds, even morepreferably less than 5 seconds, and even more preferably less than 1second. Further, the rapid cycle pressure swing adsorption units canmake use of substantially different sorbents, such as, but not limitedto, structured materials such as monoliths, laminates, and hollowfibers.

RCPSA can enable a significant increase in process intensification(e.g., higher operating frequencies and gas flow velocities) whencompared to conventional PSA. RCPSA typically utilizes a rotary valvingsystem to conduct the gas flow through a rotary adsorber module thatcontains a number of separate adsorbent bed compartments or “tubes”,each of which is successively cycled through the sorption and desorptionsteps as the rotary module completes the cycle of operations. The rotarysorber module is normally comprised of multiple tubes held between twoseal plates on either end of the rotary sorber module wherein the sealplates are in contact with a stator comprised of separate manifoldswherein the inlet gas is conducted to the RCPSA tubes and the processedpurified product gas and the tail gas exiting the RCPSA tubes areconducted away from the rotary sorber module. By suitable arrangement ofthe seal plates and manifolds, a number of individual compartments ortubes may pass through the characteristic steps of the complete cycle atany given time. In contrast, with conventional PSA, the flow andpressure variations, required for the RCPSA sorption/desorption cycle,changes in a number of separate increments on the order of seconds percycle, which smoothes out the pressure and flow rate pulsationsencountered by the compression and valving machinery. In this form, theRCPSA module includes valving elements angularly spaced around thecircular path taken by the rotating sorption module so that eachcompartment is successively passed to a gas flow path in the appropriatedirection and pressure to achieve one of the incremental pressure/flowdirection steps in the complete RCPSA cycle. One key advantage of theRCPSA technology is a significantly more efficient use of the adsorbentmaterial. The quantity of adsorbent required with RCPSA technology canbe only a fraction of that required for conventional PSA technology toachieve the same separation quantities and qualities. As a result, thefootprint, investment, and the amount of active adsorbent required forRCPSA is typically significantly lower than that for a conventional PSAunit processing an equivalent amount of gas.

The present invention may be used in PPSA, RCPSA or hybrid PPSA orRCPPSA processes where a gas or liquid is purged through the bed to helpdesorb molecules. In a PPSA process, desorption of the adsorbed speciesis accomplished by passing a gas or liquid through the contactor todesorb molecules taken up during an adsorption step. An example of a gasthat may be used is steam. In hybrid PPSA processes, the desorption ofmolecules from the contactor is accomplished by use of a thermal orpressure swing and part of the desorption is accomplished with a purge.

Improvements in the recovery of the light component are especiallyimportant for processes used to remove impurities from natural gasstreams, particularly high pressure natural gas streams. It is desirableto recover the impurities, also referred to as the “heavy component(s)”,and the methane-rich product, also referred to as the “light component”,at as high a pressure as practical for operability in natural gasprocessing. As previously mentioned, the present invention can be usedto obtain methane recovery of greater than about 80 vol. %, morepreferably greater than about 85 vol. %, even more preferably greaterthan about 90 vol. %, and most preferably greater than about 95 vol. %,even when the natural gas is fed at high pressures, such as at inletpressures greater than about 50 psig, preferably at inlet pressuresgreater than about 150 psig, more preferably at inlet pressures greaterthan about 450 psig, even more preferably at inlet pressures greaterthan about 600 psig and most preferably at inlet pressures greater thanabout 1200 psig. The present invention can be used even when the gasstream is at an exceptionally high inlet pressure of up to about 7000psig. The composition of natural gas streams directly from anunderground field (raw natural gas) will vary from field to field.Non-limiting examples of components that comprise a raw natural gasstream include water, condensates (higher molecular weight organics),methane, ethane, propane, butane, CO₂, N₂, He, H₂S, Hg, and mercaptans.Water and condensates are typically removed and the condensates sent toa petroleum refinery. In order to produce a gas that can be introducedinto a pipeline for sale to residential and commercial fuel marketscontaminants, such as N₂, Hg, mercaptans, and the acid gases CO₂ and H₂Smust to removed to acceptable levels. The levels and impurity types varyfrom gas field to gas field and in some cases can comprise the majorityof molecules in the produced gas. For example, it is not uncommon forsome natural gas fields to contain from about 0 to 90 vol. % CO₂, moretypically from about 10 vol. % to about 70 vol. % CO₂.

The present invention also provides a method to increase recovery of thelight component by conditioning the temperature and pressure of gas fedto the contactor. This method can be used with or without a contactorhaving a low volume fraction of mesopores and macropores. During theadsorptive step of well designed kinetically controlled swing adsorptionprocesses, the amount of heavy component in the micropores or freevolume can be approximately computed from the adsorption isotherm of theheavy component in equilibrium with its local gas phase concentration inthe contactor. In well designed equilibrium controlled swing adsorptionprocesses the amount of heavy component in the micropores or free volumecan be approximately computed from the competitive adsorption isothermof the heavy and light components in equilibrium with their local gasphase concentration in the contactor. These approximations are possiblebecause in well designed swing adsorption processes, the contactorprovides good mass transfer characteristics between the gas phase andthe adsorbed phase in the micropores or free volume of the contactor.The maximum attainable loading of the heavy component in the macroporesor free volume of the contactor is called q_(s) (units for q_(s) aremilli-mole/m³ of the microporous or polymeric material). At lowpressures the adsorption isotherm for the heavy component usually obeysHenry's Law and the amount of heavy component adsorbed, q_(Heavy), inthe microporous or polymeric material isq _(Heavy) =K _(Heavy) P _(Heavy) q _(s) (in milli-mole/m³)where K_(Heavy) is the Henry's constant and P_(Heavy) is the partialpressure of the heavy component. The Henry's constant, K_(Heavy) dependson temperature and usually varies according to the equation:

$K_{Heavy} = {K_{0}{\mathbb{e}}^{\frac{\Delta\; H}{RT}}\mspace{14mu}\left( {{in}\mspace{14mu}{Pascals}^{- 1}} \right)}$where K₀ is a pre-exponential factor and ΔH is the heat of adsorption(in joule/mole).

To improve selectivity and recovery for either a kinetically orequilibrium controlled swing adsorption processes the inlet temperatureand pressure should be chosen such that at the end of the adsorptionstep the loading of the heavy component in the micropores or free volumenear the point at which feed is introduced to the contactor should begreater than 0.15 q_(s) and preferably greater than 0.3 q_(s) and evenmore preferably greater than 0.6 q_(s). This requirement places a lowerbound on the inlet pressure and a maximum bound on the inlettemperature. With increasing loading of the heavy component in themicropores or free volume of the adsorbent the amount of material thatis selectively adsorbed in the contactor is increased and the amount ofmaterial that can be selectively released in the desorption step isincreased. These increases reduce the loss of the light component thatis non-selectively adsorbed into the mesopores and macropores.Increasing the loading significantly beyond this range reduces therecovery of the light component because the slope of the adsorptionisotherm tends to decrease with increasing pressure. To maximize therecovery of the light component it is also preferred that near the pointat which feed is introduced to the contactor the slope of the adsorptionisotherm for the heavy component be large enough so that:

$\frac{\partial q_{Heavy}}{\partial P_{Heavy}} > {\alpha\; K_{Heavy}\mspace{11mu} q_{s}}$where α= 1/50, more preferably α= 1/25, and even more preferably α=⅛.This inequality places a maximum bound on the inlet pressure and aminimum bound on the inlet temperature. As such these requirementsdefine a window (i.e., maxima and minima) for feed pressure andtemperature in which the recovery of the light component is optimized.Usually it is preferred to operate the swing adsorption process at thelowest pressure within the operating window as is practical. As the feedpressure decreases, the concentration of molecules in the mesopores andmacropores of the contactor decreases. Lower concentrations of moleculesnonselectively adsorbed in the mesopores and macropores leads to lowerlosses of the light component in the swing adsorption process.

This window is especially important in natural gas separations becausenatural gas is usually produced at pressures ranging from 1,500 to 7,000psi. These feed pressures are usually too high to fall within theoptimum recovery window for methane which acts as a light component inswing adsorption separation. It is possible to reduce the feed pressurewith a simple expansion nozzle, however this technique wastes energy. Itis also possible to access the optimum light component recovery windowfor separations of most heavy components (such as CO₂, N₂, H₂S, H₂O,heavy hydrocarbons, and mercaptans) by preconditioning the natural gaswith a turboexpander that recovers the energy from the gas expansion.Energy recovered from gas expansion can then be used for powergeneration or to help recompress separated acid gas components (such asCO₂ or H₂S) so that they can be disposed of in underground formations.Underground formations that are suitable for disposal /sequestration ofCO₂ and H₂S include aquifers that have a top seal that preventssignificant loss of injected acid gas components, oil reservoirs, gasreservoirs, depleted oil reservoirs and depleted gas reservoirs.Typically the separated CO₂ and H₂S has to be recompressed to pressuresgreater than 2,000 psi and often to pressures greater than 5,000 psi tobe injected into these types of underground formations. Thus, it isimportant to be able to reuse energy recovered from a turboexpander forrecompression. The cost of a turboexpander is also less than a gas firedturbine producing the same amount of power. As such, it is economicallyadvantageous to use a turboexpander to capture energy from gas expansionused to condition natural gas for the optimum methane recovery window.When a turboexpander is used, the energy can either be recovered with ashaft coupled electric generator or with a shaft coupled compressor. Itcan be advantageous to pass the gas coming out of the turboexpanderthrough a heat exchanger before introducing it into the swing adsorptionprocess in order to access the operating window that maximizes methanerecovery. Gas coming out of a turboexpander can be too cold to be in theoptimum recovery window because the work is recovered in an isentropicexpansion. Typically a heat exchanger will be run so that the gastemperature is increased before entering a swing adsorption process.These considerations are especially important when the swing adsorptionis a PSA or RCPSA process.

In applications where CO₂ is removed from natural gas in swingadsorption processes it is preferred to formulate the adsorbent with aspecific class of 8-ring zeolite materials that has a kineticselectivity. The kinetic selectivity of this class of 8-ring zeolitematerials allows CO₂ to be rapidly transmitted into zeolite crystalswhile hindering the transport of methane so that it is possible toselectively separate CO₂ from a mixture of CO₂ and methane. For theremoval of CO₂ from natural gas, this specific class of 8-ring zeolitematerials has a Si/Al ratio from about 1:1 to about 1000:1, preferablyfrom about 10:1 to about 500:1, and more from about 50:1 to about 300:1.It should be noted that as used herein, the term Si/Al is defined as themolar ratio of silica to alumina of the zeolitic structure. Thispreferred class of 8-ring zeolites that are suitable for use hereinallow CO₂ to access the internal pore structure through 8-ring windowsin a manner such that the ratio of single component diffusioncoefficients of CO₂ and methane (i.e., D_(CO2)/D_(CH4)) is greater than10, preferably greater than about 50, and more preferably greater thanabout 100 and even more preferably greater than 200. Single componentdiffusion coefficients are taken to be transport diffusion coefficientsmeasured for a pure gas in the Henry's law regime of the adsorptionisotherm. The loading of molecules in the zeolite is low in the Henry'slaw regime and in this regime the Fickian and Stephan-Maxwell diffusioncoefficients are nearly equal. The diffusivity of a porous crystallinematerial for a particular sorbate is conveniently measured in terms ofits diffusion time constant, D/r², wherein D is the Fickian diffusioncoefficient (m²/sec) and the value “r” is the radius of the crystallites(m) characterizing the diffusion distance. In situations where thecrystals are not of uniform size and geometry, r represents a meanradius representative of their corresponding distributions. One way tomeasure the time constant and diffusion coefficient is from analysis ofstandard adsorption kinetics (i.e., gravimetric uptake) using methodsdescribed by J. Crank in “The Mathematics of Diffusion”, 2nd Ed., OxfordUniversity Press, Great Britain, 1975. Another way to measure the timeconstant and diffusion coefficient is from analysis of zero lengthchromatography data using methods described by D. M. Ruthven in“Principles of Adsorption and Adsorption Processes”, John Wiley, N.Y.(1984) and by J. Kärger and D. M. Ruthven in “Diffusion in Zeolites andOther Microporous Solids”, John Wiley, N.Y. (1992). A preferred way tomeasure the time constant and diffusion coefficient is with a frequencyresponse technique described by Reyes et al. in “Frequency ModulationMethods for Diffusion and Adsorption Measurements in Porous Solids”, J.Phys. Chem. B. 101, pages 614-622, 1997. An example of a 8-ring zeolitein this class of materials that is preferred for use in swing adsorptionprocesses to remove CO₂ from natural gas is zeolite DDR. Additionalpreferred 8-ring zeolites are Sigma-1 and ZSM-58 which are zeolites thatare isotypic framework structures of DDR. At temperatures below 100° C.the single component diffusion coefficient of CO₂ is found to be morethan a hundred times greater than that of methane. From the measuredactivation energies of the diffusion coefficients, at temperatures up toabout 300° C., the diffusion coefficient of CO₂ is computed to be morethan five fold times greater than that of methane. Resistance to foulingin swing adsorption processes that remove CO₂ from natural gas isanother advantage offered by this class of 8-ring zeolite materials.

In many instances, nitrogen also has to be removed from natural gas orgas associated with the production of oil. In some cases this is becauseof the high nitrogen levels (>2 vol %) in the produced gas, and in othercases nitrogen removal is needed in order to liquefy natural gas. It mayalso be advantageous to separate nitrogen from flash gas that occurs inLNG production so that the methane and hydrocarbon products can be usedas fuel. Another application is the purification of gas from LNGboil-off so that the methane and hydrocarbon products can be recoveredor used as fuel. When recovered, it may be advantageous to re-liquefythe methane and hydrocarbon and returned them back to the LNG cargo. Inall of these applications it is desirable to selectively adsorb thenitrogen to obtain high recovery of a purified methane product fromnitrogen containing gas. There have been very few molecular sievesorbents with significant equilibrium or kinetic selectivity fornitrogen separation from methane. For N₂ separation from natural gas itis also preferred to formulate the adsorbent with a class of 8-ringzeolite materials that has a kinetic selectivity. The kineticselectivity of this class of 8-ring materials allows N₂ to be rapidlytransmitted into zeolite crystals while hindering the transport ofmethane so that it is possible to selectively separate N₂ from a mixtureof N₂ and methane. For the removal of N₂, from natural gas, thisspecific class of 8-ring zeolite materials also has a Si/Al ratio fromabout 1:1 to about 1000:1, preferably from about 10:1 to about 500:1,and more from about 50:1 to about 300:1. This preferred class of 8-ringzeolites that are suitable for use herein allow N₂ to access theinternal pore structure through 8-ring windows in a manner such that theratio of single component diffusion coefficients of N₂ and methane(i.e., D_(N2)/D_(CH4)) is greater than 5, preferably greater than about20, and more preferably greater than about 50 and even more preferablygreater than 100. Resistance to fouling in swing adsorption processesduring the remove N₂ from natural gas is another advantage offered bythis class of 8-ring zeolite materials.

In other instances, it is also desirable to remove H₂S from natural gaswhich can contain from about 0.001 vol % H₂S to about 70 vol % H₂S. Inthis case, it can be advantageous to formulate the adsorbent withstannosilicates as well as the aforementioned class of 8-ring zeolitesthat has kinetic selectivity. The kinetic selectivity of this class of8-ring materials allows H₂S to be rapidly transmitted into zeolitecrystals while hindering the transport of methane so that it is possibleto selectively separate H₂S from a mixture of H₂S and methane. For theremoval of H₂S, from natural gas, this specific class of 8-ring zeolitematerials has a Si/Al ratio from about 1:1 to about 1000:1, preferablyfrom about 10:1 to about 500:1, and more from about 50:1 to about 300:1.This preferred class of 8-ring zeolites that are suitable for use hereinallow H₂S to access the internal pore structure through 8-ring windowsin a manner such that the ratio of single component diffusioncoefficients of H₂S and methane (i.e., D_(H2S)/D_(CH4)) is greater than5, preferably greater than about 20, and more preferably greater thanabout 50 and even more preferably greater than 100. DDR frameworkzeolites, such as Sigma-1 and ZSM-58, are also suitable for the removalof H₂S from natural gas. In some applications the H₂S has to be removedto the ppm or sub-ppm levels. To achieve such extensive removal of H₂Sit can be advantageous to use a PPSA or RCPPSA process.

It is sometimes necessary to remove heavy hydrocarbons, as previouslydefined, from natural gas or gas associated with the production of oil.Heavy hydrocarbon removal may be necessary for dew point conditioningbefore the natural gas is shipped via pipeline or to condition naturalgas before it is liquefied. In other instances it may be advantageous torecover heavy hydrocarbons from produced gas in enhanced oil recovery(EOR) floods that employ CO₂ and nitrogen. In still other instances itmay be advantageous to recover heavy hydrocarbons from associated gasthat is cycled back into an oil reservoir during some types of oilproduction. In many instances where it is desirable to recover heavyhydrocarbons, the gas can be at pressures in excess of 1,000 psi and insome instances the gas pressure can be in excess of 5,000 psig, evensometimes in excess of about 7,000 psig. It is advantageous in theseapplications to use an adsorbent formulated with a zeolite having a poresize between about 5 and about 20 angstroms. Non-limiting examples ofzeolites having pores in this size range are MFI, faujasite, MCM-41 andBeta. It is preferred that the Si/Al ratio of zeolites utilized in anembodiment of a process of the present invention for heavy hydrocarbonremoval be from about 20 to about 1000, preferably from about 200 toabout 1000 in order to prevent excessive fouling of the adsorbent.

In some instances, natural gas is produced with mercaptans present andit is advantageous to use adsorption processes to aid in theirseparation. Streams containing mercaptans and components found innatural gas are present in several processes that have been developed topurify natural gas. It is possible to more selectively separatemercaptans from natural gas or natural gas components and increase therecovery of the valuable components (such as methane) using thecontactors of the present invention. It is advantageous in theseapplications to also use an adsorbent formulated with a zeolite having apore size between about 5 and about 20 angstroms. Non-limiting examplesof zeolites having pores in this size range are MFI, faujasite, MCM-41and Beta. In these applications the Si/Al ratio of the zeolite can befrom about 1 to about 1000.

The low meso and macroporous adsorbent is an integral component of thecontactors of the present invention that can be used in both equilibriumand kinetically controlled swing adsorption processes to improve lightcomponent product recovery. Adsorbents contactors of the prior artcontain significant levels of mesopores and macropores. At the end ofthe adsorption step, the mesopores and macropores, which arenon-selective, will contain significant amounts of light componentsbecause transport into the mesopores and macropores is nonselective.This is an especially important problem in high pressure RCPSA, PSA, TSAand PPSA processes because at the end of the adsorption step the numberof molecules in the mesopore and macropore spaces can be comparable tothe number of molecules selectively adsorbed in the micropores of theadsorbent. In the desorption step most of the light components containedin the mesopores and macropores are undesirably lost to the heavycomponent product stream. As such, these light molecules are notrecovered as desired with the light product. This can result insignificant loss of valuable light product. The adsorbent contactors ofthe present invention can significantly improve this recovery of lightproducts by reducing the volume fraction of the open mesopore andmacropore spaces.

In one embodiment of the present invention, the walls of the open flowparallel channels are comprised of the adsorbent. The adsorbent ispreferably a microporous adsorbent or polymer that selectively adsorbsthe heavy components. Non-limiting examples of microporous adsorbentsinclude 8-ring zeolites, titanosilicates, ferrosilicates,stannosilicates, aluminophosphates (AlPOs), silicaaluminophosphates(SAPOs) and carbon molecular sieves. In other preferred embodiments, theadsorbent material is comprised of a microporous adsorbent selected fromthe group consisting of titanosilicates and stannosilicates. In yetother preferred embodiments, the adsorbent material is comprised of amicroporous adsorbent selected from the group consisting ofaluminophosphates (AlPOs), silicaaluminophosphates (SAPOs) and carbonmolecular sieves. Preferred are zeolites for the removal of CO₂, N₂, andH₂S with the stannosilicates being more preferred for the removal ofH₂S. In other preferred embodiments, the adsorbent material is comprisedof a zeolite selected from the group consisting of MFI, faujasite,MCM-41, and Beta. Non-limiting examples of polymers that can be used asselective adsorbents include polyimides, polysulfones, andfunctionalized polymers such as amine functionalized polymers.

The adsorbent contactors of the present invention may optionally containa thermal mass (heat transfer) material to help control heating andcooling of the adsorbent of the contactor during both the adsorptionstep and desorption step of a pressure swing adsorption process. Heatingduring adsorption is caused by the heat of adsorption of moleculesentering the adsorbent. The optional thermal mass material also helpscontrol cooling of the contactor during the desorption step. The thermalmass can be incorporated into the flow channels of the contactor,incorporated into the adsorbent itself, or incorporated as part of thewall of the flow channels. When it is incorporated into the adsorbent,it can be a solid material distributed throughout the adsorbent layer orit can be included as a layer within the adsorbent. When it isincorporated as part of the wall of the flow channel, the adsorbent isdeposited or formed onto the wall. Any suitable material can be used asthe thermal mass material in the practice of the present invention.Non-limiting examples of such materials include metals, ceramics, andpolymers. Non-limiting examples of preferred metals include steel,copper, and aluminum alloys. Non-limiting examples of preferred ceramicsinclude silica, alumina, and zirconia. An example of a preferred polymerthat can be used in the practice of the present invention is polyimide.Depending upon the degree to which the temperature rise is to be limitedduring the adsorption step, the amount of thermal mass material used canrange from about 0 to about 25 times the mass of the microporousadsorbent of the contactor.

A preferred range for the amount of thermal mass in the contactor isfrom about 0 to 5 times the mass of the microporous adsorbent of thecontactor. A more preferred range for the amount of thermal massmaterial will be from about 0 to 2 times the mass of the microporousadsorbent material, most preferably from about 0 to 1 times the mass ofthe microporous material of the contactor. In a preferred embodiment, aneffective amount of thermal mass is incorporated into the contactor. Theeffective amount of thermal mass is an amount sufficient to maintain thethermal rise of the adsorbent during the adsorption step to less thanabout 100° C. In a preferred embodiment, the amount of thermal massincorporated into the contactor is an amount sufficient to maintain thethermal rise of the adsorbent during the adsorption step to less thanabout 50° C., and more preferably to less than about 10° C.

Open mesopore and macropore volume includes the volume fraction of allmesopores and macropores that are not filled with a blocking agent, andthat are non-selective and thus are capable of being occupiedessentially by all components of the gas mixture. Non-limiting examplesof blocking agents that can be used in the practice of the presentinvention include polymers, microporous materials, solid hydrocarbons,and liquids that can fill the open meso and macropore space but stillallow molecules to transport into the micropores in the selectiveadsorbent. When the blocking agent is a polymer or liquid, it ispreferred that the molecular size of the blocking agent be large enoughso that is does not significantly invade micropores of the adsorbent,but not so large that it does not fill the mesopores and macropores.When solid blocking agents are used the particle size of the solid isgreater than any selective micropores in the adsorbent but smaller thanthe meso and macropores. As such the blocking agent can fit into themeso and macropores without significantly occluding or fillingmicropores which may be present in the adsorbent.

The blocking agent fills the open meso and macropores of the adsorbentto an extent that the volume fraction of the open meso and macropores ofthe adsorbent meets the aforementioned requirements. Non-limitingexamples of polymers that can be used as blocking agents includepolyimides, polysulfones, and silicone rubbers. Non-limiting examples ofliquids that can be used as blocking agents include amines, aromaticssuch as 1,3,5 trimethylbenzene and branched saturated hydrocarbons sucha heptamethylnonane as well as liquid hydrocarbons having carbon numbersin the about 5 to about 60 range. When a liquid blocking agent is usedit is advantageous to saturate, or nearly saturate, the feed gas withthe liquid blocking agent. Non-limiting examples of solid blockingagents include hydrocarbons such as waxes and those having carbonnumbers in the 10-1000 range. Non-limiting examples of microporousmaterials that can be used in the practice of the present inventioninclude microporous carbons and zeolites having pore sizes larger thanthose of the selective structured adsorbent of this invention. Anexample of an adsorbent formulated with a blocking agent is a silica oralumina bound zeolite layer having about 30% meso and macropore volumein the interstices between the zeolite particles that is filled in witha liquid so that substantially all voids are filled with liquid (i.e.,the total resulting macro and mesoporosity in the layer is less thanabout 20%). In some cases, the blocking agent forms a continuous networkand the adsorbent is a composite structure with the microporous materialembedded within the blocking agent. A non-limiting example of such astructure is a zeolite/polymer composite where the polymer is continuousand the composite has less than about 20 volume % in open meso ormacropores.

It is also possible to formulate the adsorbent using a mesoporousmaterial that fills the macropores to reduce the overall void, or open,volume. An example of such a structure would be an adsorbent havingabout 30 volume % of macropores that are filled in with a mesoporous solgel so that the resulting mesopore and macropore volume is less thanabout 20%.

The channels, also sometimes referred to as “flow channels” or “gas flowchannels” are paths in the contactor that allow gas flow through.Generally, flow channels provide for relatively low fluid resistancecoupled with relatively high surface area. Flow channel length should besufficient to provide the mass transfer zone which is at least, afunction of the fluid velocity, and the surface area to channel volumeratio. The channels are preferably configured to minimize pressure dropin the channels. In many embodiments, a fluid flow fraction entering achannel at the first end of the contactor does not communicate with anyother fluid fraction entering another channel at the first end until thefractions recombine after exiting at the second end. It is importantthat there be channel uniformity to ensure that substantially all of thechannels are being fully utilized, and that the mass transfer zone issubstantially equally contained. Both productivity and gas purity willsuffer if there is excessive channel inconsistency. If one flow channelis larger than an adjacent flow channel, premature product break throughmay occur, which leads to a reduction in the purity of the product gasto unacceptable purity levels. Moreover, devices operating at cyclefrequencies greater than about 50 cycles per minute (cpm) requiregreater flow channel uniformity and less pressure drop than thoseoperating at lower cycles per minute. Further, if too much pressure dropoccurs across the bed, then higher cycle frequencies, such as on theorder of greater than 100 cpm, are not readily achieved.

The dimensions and geometric shapes of the parallel channel contactorsof the present invention can be any dimension or geometric shape that issuitable for use in swing adsorption process equipment. Non-limitingexamples of geometric shapes include various shaped monoliths having aplurality of substantially parallel channels extending from one end ofthe monolith to the other; a plurality of tubular members; stackedlayers of adsorbent sheets with and without spacers between each sheet;multi-layered spiral rolls, bundles of hollow fibers, as well as bundlesof substantially parallel solid fibers. The adsorbent can be coated ontothese geometric shapes or the shapes can, in many instances, be formeddirectly from the adsorbent material plus suitable binder. An example ofa geometric shape formed directly from the adsorbent/binder would be theextrusion of a zeolite/polymer composite into a monolith. Anotherexample of a geometric shape formed directly from the adsorbent would beextruded or spun hollow fibers made from a zeolite/polymer composite. Anexample of a geometric shape that is coated with the adsorbent would bea thin flat steel sheet that is coated with a microporous, low mesopore,adsorbent film, such as a zeolite film. The directly formed or coatedadsorbent layer can be itself structured into multiple layers or thesame or different adsorbent materials. Multi-layered adsorbent sheetstructures are taught in United States Patent Application PublicationNo. 2006/0169142, which is incorporated herein by reference.

The dimensions of the flow channels can be computed from considerationsof pressure drop along the flow channel. It is preferred that the flowchannels have a channel gap from about 5 to about 1,000 microns,preferably from about 50 to about 250 microns. As utilized herein, the“channel gap” of a flow channel is defined as the length of a lineacross the minimum dimension of the flow channel as viewed orthogonal tothe flow path. For instance, if the flow channel is circular incross-section, then the channel gap is the internal diameter of thecircle. However, if the channel gap is rectangular in cross-section, theflow gap is the distance of a line perpendicular to and connecting thetwo longest sides of the rectangular (i.e., the length of the smallestside of the rectangle). It should also be noted that the flow channelscan be of any cross-sectional configuration. Preferred embodiments arewherein the flow channel cross-sectional configuration is eithercircular, rectangular or square. However, any geometric cross-sectionalconfiguration may be used, such as but not limited to, ellipses, ovals,triangles, or various polygonal shapes. In other preferred embodiments,the ratio of the adsorbent volume to flow channel volume in theadsorbent contactor is from about 0.5:1 to about 100:1, and morepreferably from about 1:1 to about 50:1.

In some RCPSA applications, the flow channels are formed when adsorbentsheets are laminated together. Typically, adsorbent laminates for RCPSAapplications have flow channel lengths from about 0.5 centimeter toabout 10 meter, more typically flow channel lengths from about 10 cm toabout 1 meter and a channel gap of about 50 to about 250 microns. Thechannels may contain a spacer or mesh that acts as a spacer. Forlaminated adsorbents, spacers can be used which are structures ormaterial, that define a separation between adsorbent laminates.Non-limiting examples of the type of spacers that can be used in thepresent invention are those comprised of dimensionally accurate:plastic, metal, glass, or carbon mesh; plastic film or metal foil;plastic, metal, glass, ceramic, or carbon fibers and threads; ceramicpillars; plastic, glass, ceramic, or metal spheres, or disks; orcombinations thereof. Adsorbent laminates have been used in devicesoperating at PSA cycle frequencies up to at least about 150 cpm. Theflow channel length may be correlated with cycle speed. At lower cyclespeeds, such as from about 20 to about 40 cpm, the flow channel lengthcan be as long as or longer than one meter, even up to about 10 meters.For cycle speeds greater than 40 cpm, the flow channel length typicallyis decreased, and may vary from about 10 cm to about 1 meter. Longerflow channel lengths can be used for slower cycle PSA processes. Rapidcycle TSA processes tend to be slower than rapid cycle PSA processes andas such longer flow channel lengths can also be used with TSA processes.

The overall adsorption rate of the swing adsorption processes ischaracterized by the mass transfer rate from the flow channel into theadsorbent. It is desirable to have the mass transfer rate of the speciesbeing removed (i.e., the heavy component) high enough so that most ofthe volume of the adsorbent is utilized in the process. Since theadsorbent selectively removes the heavy component from the gas stream,inefficient use of the adsorbent layer can lower recovery of the lightcomponent and/or decrease the purity of the light product stream. Withuse of the present invention, it is possible to formulate an adsorbentwith a low volume fraction of meso and macroporous such that most of thevolume of the adsorbent, which will be in the microporous range, isefficiently used in the adsorption and desorption of the heavycomponent. One way of doing this is to have an adsorbent ofsubstantially uniform thickness where the thickness of the adsorbentlayer is set by the mass transfer coefficients of the heavy componentand the time of the adsorption and desorption steps of the process. Thethickness uniformity can be assessed from measurements of the thicknessof the adsorbent or from the way in which it is fabricated. It ispreferred that the uniformity of the adsorbent be such that the standarddeviation of its thickness is less than about 25% of the averagethickness. More preferably, the standard deviation of the thickness ofthe adsorbent is less than about 15% of the average thickness. It iseven more preferred that the standard deviation of the adsorbentthickness be less than about 5% of the average thickness.

Calculation of these mass transfer rate constants is well known to thosehaving ordinary skill in the art and may also be derived by those havingordinary skill in the art from standard testing data. D. M. Ruthven & C.Thaeron, Performance of a Parallel Passage Absorbent Contactor,Separation and Purification Technology 12 (1997) 43-60, which isincorporated herein by reference, clarifies many aspects of how the masstransfer is affected by the thickness of the adsorbent, channel gap andthe cycle time of the process. Also, U.S. Pat. No. 6,607,584 to Moreauet al., which is also incorporated by reference, describes the detailsfor calculating these transfer rates and associated coefficients for agiven adsorbent and the test standard compositions used for conventionalPSA.

A figure of merit for the mass transfer through the adsorbent layer is atime constant, τ_(a), for transport of the heavy component computed ateach point in the adsorbent. For a planar adsorbent sheet with thicknessin the x direction, and the y and z directions being in the plane of thesheet, the time constant τ_(a) of the heavy component isτ_(a) [x,y,z]=Minimum[L _(path) ² /D _(path)](in seconds)where D_(path) is the average transport diffusion coefficient of theheavy component along a path from the feed channel to the point (x,y,z)and L_(path) is the distance along the path. There are many possibletrajectories or paths from the feed channel to each point (x,y,z) in theadsorbent. The time constant is the minimum of the possible timeconstants (L_(path) ²/D_(path)) along all possible paths from the feedchannel to the (x,y,z) point in the adsorbent. This includes pathsthrough meso and macropores. If there is a solid material in theadsorbent (such as that which may be included for heat management) therewill be no transport within it and (x,y,z) points within it are notincluded in the computation. The transport diffusion coefficient of eachspecies is taken to be the single component Stefan-Maxwell diffusioncoefficient for each species. The average transport diffusioncoefficient along the path, D_(path), is the linearly averaged diffusioncoefficient along the path. A linear averaging is sufficient to providea diffusion coefficient characterizing the path. When the heavycomponent has many species the diffusion coefficient, D_(path), is alsocompositionally averaged. The diffusion coefficient depends ontemperature and it may depend on pressure as well. To the extent thatthe diffusion coefficient changes, it must be averaged for thetemperature and pressure changes occurring during a cycle. For anadsorbent to be efficient, the averaged thickness of the adsorbent layerpreferably is chosen such that the time constant for at least half thepoints (or volume) in the adsorbent that is not a dense solid is lessthan the cycle time of the process. More preferably, the averagethickness of the adsorbent layer is chosen such that the time constantfor at least 75% of the points (or volume) in the adsorbent that is nota dense solid is less than the cycle time of the process. Even morepreferably the average thickness of the adsorbent layer is chosen suchthat the time constant for at least 75% of the points (or volume) in theadsorbent that is not a dense solid is less than about 25% of the cycletime of the process.

The present invention can be applied to improve the separation ofmolecular species from synthesis gas. Synthesis gas can be produced by awide variety of methods, including steam reforming of hydrocarbons,thermal and catalytic partial oxidation of hydrocarbons, and many otherprocesses and combinations known in the art. Synthesis gas is used in alarge number of fuel and chemical applications, as well as powerapplications such as Integrated Gasification Combined Cycle (IGCC). Allof these applications have a specification of the exact composition ofthe syngas required for the process. As produced, synthesis gas containsat least CO and H₂. Other molecular components in the gas can be CH₄,CO₂, H₂S, H₂O, and N₂. Minority (or trace) components in the gas caninclude hydrocarbons, NH₃ and NOx. In almost all applications most ofthe H₂S has to be removed from the syngas before it can be used and inmany applications it is desirable to remove much of the CO₂. Inapplications where the syngas is used as a feedstock for a chemicalsynthesis process, it is generally desirable to adjust the H₂/CO ratioto a value that is optimum for the process. In certain fuelapplications, a water-gas shift reaction may be employed to shift thesyngas almost entirely to H₂ and CO₂, and in many such applications itis desirable to remove the CO₂.

The present invention provides a method for increasing the recovery ofthe valuable molecular components from synthesis gas. In mostapplications valuable components are CO and H₂. When multiple speciesare removed from the synthesis gas, individual contactors, eachoptimized for the removal of a particular component, can be used.Multiple contactors can be used because the invention provides a meansof rapidly changing the pressure in the contactor allowing for rapidcycle operation and consequentially small equipment size. Alternativelyseveral different adsorbents can be incorporated into a singlecontactor. This provides a means of selectively removing several specieswith a single contactor.

It can be desirable to recover separated acid gases, such as H₂S and/orCO₂, at higher pressure. The recovery of higher pressure acid gases canbe desirable, for example, when CO₂ sequestration is planned. In thesecases, adsorption by temperature swing (TSA) can be preferred overpressure swing. The invention provides a means to rapidly change thecontactor temperature without experiencing large heat losses, longheat-up and cool-down times, or adsorbate dilution. Temperature swingadsorption can be executed with fixed parallel-channel contactors andassociated valves, or by means of a rotary-based parallel-channelcontactor following the approach of a Ljungstrom heat exchanger.

Rapid TSA cycle operation is facilitated with a parallel channelcontactor where the adsorbent is on one surface of a compact heatexchange structure. Heating and cooling would take place in a channelisolated from the adsorbing and desorbing material. In thisconfiguration, a thermal wave can be made to move through the contactorduring the adsorption step allowing for better separation of adsorbedcomponents. In some instance a chromatographic like separation can beachieved (with no dilution from a carrier gas). This type of parallelchannel contactor arrangement can be extremely energy efficient. Thermalenergy used in the swing adsorption process can be readily recovered andreused. Because of the energy efficiency a larger degree of thermalswing can be used.

The contactors of the present invention can better be understood withreference to the Figures hereof. FIG. 1 hereof is a representation of aparallel channel contactor of the present invention in the form of amonolith formed directly from a microporous adsorbent plus binder andcontaining a plurality of parallel flow channels. A wide variety ofmonolith shapes can be formed directly by extrusion processes. Anexample of a cylindrical monolith 1 is shown schematically in FIG. 1hereof. The cylindrical monolith 1 contains a plurality of parallel flowchannels 3. These flow channels 3 can have channel gaps from about 5 toabout 1,000 microns, preferably from about 50 to about 250 microns, aslong as all channels of a given contactor have substantially the samesize channel gap. The channels can be formed having a variety of shapesincluding, but not limited to, round, square, triangular, and hexagonal.The space between the channels is occupied by the adsorbent 5. As shownthe channels 3 occupy about 25% of the volume of the monolith and theadsorbent 5 occupies about 75% of the volume of the monolith. Theadsorbent 5 can occupy from about 50% to about 98% of the volume of themonolith. The effective thickness of the adsorbent can be defined fromthe volume fractions occupied by the adsorbent 5 and channel structureas:

${{Effective}\mspace{14mu}{Thickness}\mspace{14mu}{Of}\mspace{14mu}{Adsorbent}} = {\frac{1}{2}\mspace{11mu}{Channel}\mspace{14mu}{Diameter}\mspace{11mu}\frac{{Volume}\mspace{14mu}{Fraction}\mspace{14mu}{Of}\mspace{14mu}{Adsorbent}}{{Volume}\mspace{14mu}{Fraction}\mspace{14mu}{Of}\mspace{14mu}{Channels}}}$

For the monolith of FIG. 1 hereof the effective thickness of theadsorbent will be about 1.5 times the diameter of the feed channel. Whenthe channel diameter is in a range from about 50 to about 250 microns itis preferred that the thickness of the adsorbent layer, in the casewherein the entire contactor is not comprised of the adsorbent, be in arange from about 25 to about 2,500 microns. For a 50 micron diameterchannel, the preferred range of thickness for the adsorbent layer isfrom about 25 to about 300 microns, more preferred range from about 50to about 250 microns. FIG. 2 is a cross-sectional view along thelongitudinal axis showing feed channels 3 extending through the lengthof the monolith with the walls of the flow channels formed entirely fromadsorbent 5 plus binder. A schematic diagram enlarging a small crosssection of the feed channels 3 and adsorbent layer 5 of FIG. 2 is shownin FIG. 3 hereof. The adsorbent layer is comprised of a microporousadsorbent, or polymeric, particles 7; solid particles (thermal mass) 9;that act as heat sinks, a blocking agent 13 and open mesopores andmicropores 11. As shown, the microporous adsorbent or polymericparticles 7 occupy about 60% of the volume of the adsorbent layer andthe particles of thermal mass 9 occupy about 5% of the volume. With thiscomposition, the voidage (flow channels) is about 55% of the volumeoccupied by the microporous adsorbent or polymeric particles. The volumeof the microporous adsorbent 5 or polymeric particles 7 can range fromabout 25% of the volume of the adsorbent layer to about 98% of thevolume of the adsorbent layer. In practice, the volume fraction of solidparticles 9 used to control heat will range from about 0% to about 75%,preferably about 5% to about 75%, and more preferably from about 10% toabout 60% of the volume of the adsorbent layer. A blocking agent 13fills the desired amount of space or voids left between particles sothat the volume fraction of open mesopores and macropores 11 in theadsorbent layer 5 is less than about 20%.

When the monolith is used in a gas separation process that relies on akinetic separation (predominantly diffusion controlled) it isadvantageous for the microporous adsorbent or polymeric particles 7 tobe substantially the same size. It is preferred that the standarddeviation of the volume of the individual microporous adsorbent orpolymeric particles 7 be less than 100% of the average particle volumefor kinetically controlled processes. In a more preferred embodiment thestandard deviation of the volume of the individual microporous adsorbentor polymeric particles 7 is less than 50% of the average particlevolume. The particle size distribution for zeolite adsorbents can becontrolled by the method used to synthesize the particles. It is alsopossible to separate pre-synthesized microporous adsorbent particles bysize using methods such as a gravitational settling column. It may alsobe advantageous to use uniformly sized microporous adsorbent orpolymeric particles in equilibrium controlled separations.

There are several ways that monoliths can be formed directly from astructured microporous adsorbent. For example, when the microporousadsorbent is a zeolite, the monolith can be prepared by extruding anaqueous mixture containing effective amounts of a solid binder, zeoliteand adsorbent, solid heat control particles, and polymer. The solidbinder can be colloidal sized silica or alumina that is used to bind thezeolite and solid heat control particles together. The effective amountof solid binder will typically range from about 0.5 to about 50% of thevolume of the zeolite and solid heat control particles used in themixture. If desired, silica binder materials can be converted in a postprocessing step to zeolites using hydrothermal synthesis techniques and,as such, they are not always present in a finished monolith. A polymeris optionally added to the mixture for rheology control and to givegreen extrudate strength. The extruded monolith is cured by firing it ina kiln where the water evaporates and the polymer burns away, therebyresulting in a monolith of desired composition. After curing themonolith, the adsorbent layer 5 will have about 20 to about 40 vol. %mesopores and macropores. A predetermined amount of these pores can befilled with a blocking agent 13, as previously discussed, in asubsequent step such as by vacuum impregnation.

Another method by which a monolith can be formed directly from amicroporous adsorbent is by extruding a polymer and microporousadsorbent mixture. Preferred microporous adsorbents for use in extrusionprocess are carbon molecular sieves and zeolites. Non-limiting examplesof polymers suitable for the extrusion process include epoxies,thermoplastics, and curable polymers such as silicone rubbers that canbe extruded without an added solvent. When these polymers are used inthe extrusion process, the resulting product will preferably have a lowvolume fraction of meso and macropores in the adsorbent layer.

FIG. 4 hereof is a representation of a parallel channel contactor 101 ofthe present invention in the form of a coated monolith where anadsorbent layer is coated onto the walls of the flow channels of apreformed monolith. For the parallel channel contactors of this Figure,an extrusion process is used to form a monolith from a suitablenon-adsorbent solid material, preferably a metal such as steel, aceramic such as cordierite, or a carbon material. By the term“non-adsorbent solid material” we mean a solid material that is not tobe used as the selective adsorbent for the parallel channel contactor.An effective amount and thickness of a ceramic or metallic glaze, or solgel coating, 119 is preferably applied to effectively seal the channelwalls of the monolith. Such glazes can be applied by slurry coating thechannel walls, by any suitable conventional means, followed by firingthe monolith in a kiln.

Another approach is to apply a sol gel to the channel walls followed byfiring under conditions that densify the coating. It is also possible touse vacuum and pressure impregnation techniques to apply the glaze orsol gel to the channel walls. In such a case, the glaze or sol gel willpenetrate into the pore structure of the monolith 117. In all cases, theglaze seals the wall of the channel such that gas flowing thorough thechannel is not readily transmitted into the body of the monolith. Anadsorbent layer 105 is then uniformly applied onto the sealed walls ofthe channels. The adsorbent layer 105 reduces the opening, or bore, ofthe channels, thus the flow channel 103 used in swing adsorptionprocesses is the open channel left inside of the coating. These flowchannels 103 can have channel gaps as previously defined. The adsorbentlayer 105 can be applied as a coating, or layer, on the walls of theflow channels by any suitable method. Non-limiting examples of suchmethods include fluid phase coating techniques, such as slurry coating,slip coating, hydrothermal film formation, hydrothermal coatingconversion, and hydrothermal growth. When non-hydrothermal coatingtechniques are used, the coating solutions should include at least themicroporous adsorbent or polymeric particles, a viscosifying agent suchas polyvinyl alcohol, heat transfer (thermal mass) solids, andoptionally a binder. The heat transfer solid may not be needed becausethe body of the monolith 101 can act to as its own heat transfer solidby storing and releasing heat in the different steps of the separationprocess cycle. In such a case, the heat diffuses through the adsorbentlayer 105 and into the body of the monolith 101. If a viscosifyingagent, such as polyvinyl alcohol, is used it is usually burns away whenthe coating is cured in a kiln. It can be advantageous to employ abinder such as colloidal silica or alumina to increase the mechanicalstrength of the fired coating. Mesopores or macropores will typicallyoccupy from about 20 to about 40% of the volume of the cured coating. Aneffective amount of blocking agent is applied to complete the adsorbentlayer for use. By effective amount of blocking agent we mean that amountneeded to occupy enough of the mesopores and macropores such that theresulting coating contains less than about 20% of its pore volume inopen mesopores and macropores.

If a hydrothermal film formation method is employed, the coatingtechniques used can be very similar to the way in which zeolitemembranes are prepared. An example of a method for growing a zeolitelayer is taught in U.S. Pat. No. 7,049,259, which is incorporated hereinby reference. Zeolite layers grown by hydrothermal synthesis on supportsoften have cracks and grain boundaries that are mesopore and macroporein size. The volume of these pores is often less than about 10 volume %of the film thickness and there is often a characteristic distance, orgap, between cracks. Thus, as-grown films can often be used directly asan adsorbent layer without the need for a blocking agent. Examples ofcrack and grain boundaries in as-grown zeolite films are shown in highresolution scanning electron micrographs FIGS. 5 and 6 hereof. Thezeolite film of FIG. 5 is comprised of Sigma-1 zeolite which has aframework structure that is isotypic with DDR. The film is about 25micrometers thick with cracks 151 that are about 100 to about 300angstrom wide, which cracks are readily visible on the surface of thefilm. The zeolite film in FIG. 6 is an MFI film that was produced bycoating a first coating layer approximately 0.5 micrometers thick usingcolloidal ZSM-5 seeds onto a support and then placing the seed coveredsupport in a hydrothermal synthesis solution. The colloidal ZSM-5 seedsnucleated the growth of a MFI film about 15 micrometers thick in thehydrothermal synthesis step. The Si/Al ratio of the deposited film wasgreater than about 100. Cracks of several hundred angstrom size 153 andgaps 155 between the MFI zeolite crystals are apparent in themicrograph. Besides the cracks and gaps there are grain boundariesbetween crystals. These grain boundaries can connect to the crackstructure and aid in transport of molecules to the zeolite crystals thatare deeper in the film. Many of the grain boundaries have dimensions inthe micropore range and some have dimensions in the mesopore range. Itis apparent from the micrographs of FIGS. 5 and 6 that the openmesopores and macropores occupy a very small amount of the volume at thesurface of the film. Cross sectional images of these films confirmedthat the open meso and macropore in fact do occupy less than about 7% ofthe volume of the films.

FIG. 7 hereof is a representation of a parallel channel contactor of thepresent invention in the form of a coated monolith 201 for TSAapplications where the adsorbent layer is coated onto the channel of apreformed monolith comprised of non-adsorbent material. When TSA orRCTSA processes are performed the contactor will preferably have paths,or separate channels, that can be used to heat and cool the adsorbent.For TSA or RCTSA processes, the parallel channel contactor can beconfigured in a configuration similar to a shell and tube heat exchangerwith the adsorbent coated on the tube walls of the heat exchanger. Inthis Figure, an extrusion process is used to form a monolith from asuitable non-adsorbent material including a metal such as steel, or aceramic such as cordierite, or a carbon. A ceramic or metallic glaze orsol gel coating 219 is applied to seal the channel walls of themonolith. As previously mentioned, such glazes can be applied by slurrycoating the channel walls followed by curing by firing. A sol gel canalso be applied to the channel walls and then fired under conditionsthat densify the coating. As previously mentioned, it is also possibleto use vacuum and pressure impregnation techniques to apply the glaze orsol gel. In this case the glaze or sol gel will penetrate into the porestructure of the monolith 217. In all cases the glaze seals the wall ofthe channel such that gas flowing thorough the channel is not readilytransmitted into the body of the monolith. It may also be desirable toimpregnate the pore structure of the monolith 217 with a solid materialbefore the channel walls are sealed. Alternate rows of channels aresealed at their ends 215 in order to provide for TSA operation. At theopposite end of the monolith these same rows of channels are alsosealed. Slots (223 and 225) are cut through the monolith at both ends toprovide flow access to the sealed rows of channels 215. Sealing surfaces219 are provided at both ends of the monolith as well as in the middleof the monolith 221. In operation, the monolith will be mounted in amodule in a manner that seals the ends of the channels as well as themiddle of the monolith. Any suitable technology can be used to seal theends of the channels including metallic welds, gasketing with materialssuch as rubbers or carbons, and the use of adhesives such as inorganiccements and epoxies. The module is configured so that a heating orcooling fluid can be flowed through the channels sealed at the ends 215by introducing it through the slots 223 and removing it through slots225. The heating and cooling fluid will undergo heat exchange with fluidflowing through the channels that are open at the end of the module.These modifications to the monolith convert it into a heat exchanger. Itwill be understood that there are various other ways in which heatexchangers can be produced or configured. Non-limiting examples of suchother ways include shell and tube heat exchangers, fiber film heatexchangers and printed circuit heat exchangers, all of which are wellknown in the art. By coating an adsorbent layer with a low volumefraction of meso and macropores on one side of a heat exchanger it canbe used in accordance with the present invention. As such, this exampleillustrates how heat exchangers can be converted into modules suitablefor TSA with an adsorbent layer having a low volume fraction of meso andmacropores.

Feed channels 203 can have channel gaps from about 5 to about 1,000microns, preferably from about 50 to about 250 microns. When the channelgap 203 is in a range from 50 to about 250 microns it is preferred thatthe thickness of the adsorbent layer 205 be in a range form about 25 toabout 2,500 microns. For a 50 micron channel gap 203 the preferred rangeof thickness for the adsorbent layer is from 25 to 300 microns and amore preferred range is from 50 to 250 microns. The techniquespreviously discussed above can be used to coat the adsorbent layer intothe monolith.

FIG. 8 hereof is a schematic of a parallel channel contactor of thepresent invention in the form of a substantially parallel array ofhollow fibers embedded in a matrix material 331. A wide variety ofhollow fibers can be formed directly using conventional spinning andextrusion processes. The contactor of FIG. 8 is formed from an array ofhollow fibers 301. The bores 303 of the fibers are used as flowchannels. These flow channels 303 can also have channel gaps from about5 to about 1,000 microns, preferably from about 50 to about 250 micronsas previously mentioned. Also as previously mentioned, the walls of thefibers contains an adsorbent layer 305. When the channel gap 303 is in arange from 50 to about 250 microns it is preferred that the thickness ofthe adsorbent layer 305 be in a range from about 25 to 2,500 microns.

Various different methods known in the art can be used to produce theadsorbent layer 305 in the fiber. For example, the hollow polymer fiberswith low mesoporosity can be extruded. Some spinning techniques can alsobe used to produce hollow fibers with mesopores and macropores that canbe removed in post treatments such as thermal annealing, polymercoating, epoxy coating or filling with a blocking agent. Hollow fibersthat are composites of polymers and adsorbents can be formed in bothspinning and extrusion processes. These processes often form the fiberfrom a dope containing the polymer, adsorbent particles, and often asolvent. In some cases, the surface of the adsorbent particle isfunctionalized to promote adhesion between the polymer matrix and theadsorbent particle. When the volume fraction of meso and macropores istoo high, it can be lowered by a post treatment using thermal annealing,or by filling an effective amount of meso and macropores with a blockingagent.

It is also possible to produce hollow fibers of zeolites by extrusion.In these processes the zeolite is mixed with a polymer or an oligomericviscosifying agent, such as a lower molecular weight polyvinyl alcohol.Optionally, a solvent such as water, alcohol, or liquid hydrocarbon canbe added to the dope. It is also optional to use a binder material, suchas colloidal silica or colloidal alumina that can be added to this dope.Solid particles, such as alumina or aluminum can also be added to thedope. The dope is then extruded and from the green state the finalceramic body is produced. This fiber, in the green state, can then beplaced into a kiln and fired to form the final fiber comprised ofzeolite, and optionally binder and solid particles. Alternatively, thefiber in the green state can be placed in a hydrothermal synthesisreactor to produce a final fiber comprised of zeolite, and optionallybinder and solid thermal mass particles. Another method to produce azeolite fiber is by hydrothermally growing a zeolite coating on a solidpolymer fiber that burns away during the calcinations step. In allcases, mesoporosity and macroporosity in the fiber can be reduced towithin a target range by filling with a blocking agent in a subsequentstep.

The fibers can be formed into a substantially parallel array to form acontactor of the present invention. One method to do this is with anembedding or potting process that surrounds the entire length of thefibers with a matrix material 325. To visualize the array in FIG. 8 theend of the matrix material 351 has been rendered transparent along withthe face 321 of the embedded hollow fiber bundle. In many instances, itcan be advantageous to coat the exterior of the fiber with a materialthat acts as a diffusion barrier 315. Non-limiting examples of materialsthat can act as diffuision barriers include sputter deposited metal andceramic films, evaporated metal and ceramic films, metal and ceramicfilms formed by chemical vapor deposition, coated composites of polymersand solids (such as clays) and coatings of polymers that have lowdiffusion coefficients. To act as a diffusion barrier, the effectivediffusion coefficient of the coating should be less than about 1/10 theaverage diffusion coefficient in the adsorbent layer and preferably lessthan about 1/1000 the average diffusion coefficient in the adsorbentlayer. When a diffusion barrier is used, the gas in the feed channel iseffectively contained in the feed channel and adsorbent layer. This caneliminate the need for a supporting matrix around the fibers, thuslowering the mass of the contactor, and in some cases allowing for thecycle time in the process to be decreased (i.e., rapid cycle operation).

Another fabrication method suitable for use herein is to coat theadsorbent inside the prefabricated fiber such as a hollow glass fiber,hollow silica fiber or hollow polymer fiber. Coating methods previouslydescribed can be used to form an adsorbent layer inside of aprefabricated fiber. When the prefabricated fiber is made from glass, orsilica, the final product has a built in diffusion barrier 315.

When there is no diffusion barrier on the fiber it is advantageous forthe matrix material to contain an adsorbent having a low volume fractionof meso and macropores. In this case, it is advantageous to space thefibers closely together with the distance between adjacent fibers lessthan about 5 fiber diameters, preferably less than about 1.5 fiberdiameters. When there is a diffusion barrier on the outer surface of thefibers, it can be advantageous to embed only the ends 351 and 353 of thefiber bundle in the matrix material. In this case, the matrix materialonly has to support the fibers and not have substantial gas flow throughthe material. It can be composed of polymer, metal or ceramic orcombinations thereof. It is preferred that the matrix be nonporous andrequirements for having an adsorbent in the matrix material can berelaxed or eliminated. Requirements for spacing between fibers can beless critical than when the entire length of the fiber is potted orembedded. The matrix material can be applied selectively to the ends ofthe fiber bundles by any suitable method known in the art. Non-limitingexamples of such methods include potting, embedding, casting,electroplating, or electroless plating processes. To avoid plugging theend of the fibers the end of the fibers can be filled with a materialthat can be readily removed after the matrix is applied. Non-limitingexamples of materials that can be readily removed include polymers,metals, salts and inorganics that can be selectively dissolved or etchedaway after the matrix material has been applied. Grinding, machining andpolishing methods can also be used to open the ends of the fibers. Othermethods to pot or embed the ends of the fibers are similar to those usedto form hollow fiber membrane modules. When the ends of the fiber bundleare potted with a matrix material it is advantageous to place thecontactor into an operational PSA, RCPSA, RCPPSA or PPSA module in amanner such that most of the feed gas flows through the bore of thefiber. One way to ensure that the flow goes through the bore of thefiber is to place a fiberous packing, or inram, between the matrixmaterial at the ends 351 and 353 and the interior of the PSA, RCPSA,RCPPSA or PPSA module. Another way is to bond the ends of the contactorto the interior of the pressure swing adsorption module.

FIGS. 9 and 10 hereof are representations of a parallel channelcontactor of the present invention in the form of a hollow fibercontactor for a TSA process where the adsorbent layer 405 comprises partof the wall of the fiber with a center feed channel 403. In FIG. 10, theouter surfaces of the housing for the contactor 401 are renderedtransparent with only dotted lines indicating the edges of the outersurface. The hollow fibers used in this example have a diffusion barrier415 on their exterior surface and can be manufactured using techniquesdescribed for FIG. 4 hereof. The ends of the fiber bundle are potted orembedded in a matrix material 417. This potted array is then sealed intoa tubular housing 401. Sealing surfaces 419 are provided at the ends ofthe tubular housing 401. A sealing surface 421 is also provided in themiddle of the housing. Slots 423 and 425 are cut through the wall nearthe ends of the tubular housing to allow for the flow of heating andcooling fluids.

In operation, the tubular housing is mounted in a TSA or RCTSA module ina manner that seals the ends of the channels as well as the middle ofthe monolith. Any suitable sealing technology can be used. Non-limitingexamples of sealing technologies that can be used in the practice of thepresent invention include metallic welds, gasketing with materials suchas rubbers or carbons, and the use of adhesives such as inorganiccements or epoxies. The module is configured so that a heating orcooling fluid can be flowed inside the hollow tubular housing 401 byintroducing it through slots 423 and removing it through slots 425. Theheating and cooling fluid will undergo heat exchange with fluid flowingthrough the hollow fibers which are open at the end of the module. Withthese sealing arrangements, the tubular housing 401 containing theparallel array of hollow fibers becomes a heat exchanger suitable foruse in TSA processes. The fibers have an adsorbent layer 405 with a lowvolume fraction of meso and macropores.

FIG. 11 hereof is a representation of a parallel channel contactor ofthe present invention in which the parallel channels are formed fromlaminated sheets containing adsorbent material. Laminates, laminates ofsheets, or laminates of corrugated sheets can be used in PSA RCPSA, PPSAor RCPPSA processes. Laminates of sheets are known in the art and aredisclosed in US Patent Applications US20060169142 A1 and U.S. Pat. No.7,094,275 B2 which are incorporated herein by reference. When theadsorbent is coated onto a geometric structure or components of ageometric structure that are laminated together, the adsorbent can beapplied using any suitable liquid phase coating techniques. Non-limitingexamples of liquid phase coating techniques that can be used in thepractice of the present invention include slurry coating, dip coating,slip coating, spin coating, hydrothermal film formation and hydrothermalgrowth. When the geometric structure is formed from a laminate, thelaminate can be formed from any material to which the adsorbent of thepresent invention can be coated. The coating can be done before or afterthe material is laminated. In all these cases the adsorbent is coatedonto a material that is used for the geometric shape of the contactor.Non-limiting examples of such materials include glass fibers, milledglass fiber, glass fiber cloth, fiber glass, fiber glass scrim, ceramicfibers, metallic woven wire mesh, expanded metal, embossed metal,surface-treated materials, including surface-treated metals, metal foil,metal mesh, carbon-fiber, cellulosic materials, polymeric materials,hollow fibers, metal foils, heat exchange surfaces, and combinations; ofthese materials. Coated supports typically have two major opposingsurfaces, and one or both of these surfaces can be coated with theadsorbent material. When the coated support is comprised of hollowfibers, the coating extends around the circumference of the fiber.Further support sheets may be individual, presized sheets, or they maybe made of a continuous sheet of material. The thickness of thesubstrate, plus applied adsorbent or other materials (such as desiccant,catalyst, etc.), typically ranges from about 10 micrometers to about2000 micrometers, more typically from about 150 micrometers to about 300micrometers.

Metallic mesh supports can provide desirable thermal properties of highheat capacity and conductivity which “isothermalize” a PSA, RCPSA, PPSAor RCPPSA cycle to reduce temperature variations that degrade theprocess when conducted under more adiabatic conditions. Also, metalfoils are manufactured with highly accurate thickness dimensionalcontrol. The metal foil may be composed of, without limitation,aluminum, steel, nickel, stainless steel or alloys thereof. Hence thereis a need for a method to coat metal foils with a thin adsorbent layerof accurately controlled thickness, with necessary good adhesion. Onemethod for doing this is by hydrothermal synthesis. Coating proceduresused can be very similar to the way in which zeolite membranes areprepared as discussed above. Zeolite layers grown by hydrothermalsynthesis on supports often have cracks which are meso and micropores.Examples of these cracks have been shown in FIGS. 5 and 6 hereof. Thevolume of these pores is often less than about 10 volume % of the filmthickness and there is often a characteristic distance between cracks.Another method of coating a metal foil is with thick film coating isslip casting, or doctor blading. An aqueous slurry of prefabricatedzeolite particles, binder (for example colloidal silica or alumina),viscosifying agent such as a polymer like polyvinyl alcohol is cast forexample onto a metal foil and fired to remove the polymer and cure thebinder and zeolite. The product, after firing, is then a bound zeolitefilm on a metal foil typically containing about 30 to about 40 volume %voids. To make a suitable adsorbent layer, the voids are filled in asubsequent step by coating the bound zeolite film with a polymer or byintroducing a liquid into the voids of the bound zeolite film. The finalproduct, after filling the voids with a polymer or liquid, will be anadsorbent layer having the low meso and macroporosity requirements ofthe present invention.

Another method for coating metal foils with prefabricated zeolitecrystals, or microporous particles, is electrophoretic deposition (EPD).EPD is a technique for applying high quality coatings of uniformthickness to metal substrates. The method can be used to apply organicand inorganic particulate coatings on electrically conductivesubstrates. Slurry compositions containing prefabricated zeolites, ormicroporous particles, may be electrophoretically applied to a rigidsupport material, such as by using the method described in Bowie Keeferet al.'s prior Canadian patent application No. 2,306,311, entitled“Adsorbent Laminate Structure,” which is incorporated herein byreference.

Some contactor geometric shapes will require that the adsorbent beapplied to the channel surface in a layer using a colloidal bindermaterial or that an entire geometric shape be comprised of the adsorbentplus colloidal binder and containing a plurality of parallel channels.When a colloidal binder is used, the selection of the colloidal materialdepends on the particular adsorbent used. Colloidal materials capable offunctioning as a binder and/or which form a gel are preferred. Suchcolloidal materials include, without limitation, colloidal silica-basedbinders, colloidal alumina, colloidal zirconia, and mixtures ofcolloidal materials. “Colloidal silica” refers to a stable dispersion ofdiscrete amorphous silicon dioxide particles having a particle sizeranging from about 1 to about 100 nanometers. Suitable colloidal silicamaterials also can be surface modified, such as by surface modificationwith alumina. Another type of colloidal binder suitable for use hereininclude clay materials, such as palygorskite (also known asattapulgite), which are hydrated magnesium aluminum silicates. Also,inorganic binders may be inert; however, certain inorganic binders, suchas clays, used with zeolite adsorbents may be converted in-situ fromkaolin binders to zeolite so that the zeolite is self-bound with minimalinert material. In these bound structures, the voids between thecolloidal particles form mesopores and the voids between the adsorbentparticles form meso and/or macropores. A blocking agent can be appliedto fill the majority of the meso and macroporosity in these bound layersso that the adsorbent meets the open pore volume requirement of thisinvention. Organic binders used to bind activated carbon particulates inlaminated structures may be pyrolyzed to form a useful carbonaceousadsorbent.

FIG. 11 hereof illustrates an exploded view of an embodiment of thepresent invention wherein a microporous adsorbent film 505, preferablycomprising DDR, is hydrothermally grown on each of both faces of flatmetal foils 509, which is preferably fabricated from a corrosionresistant metal such as stainless steel. The separate metal foils 509with the adsorbent films 505 are fabricated to form a parallel channelcontactor 501. Spacers of appropriate size may placed between the metalfoils during contactor fabrication so that the channel gap 503 is of apredetermined size. Preferably about half of the volume of the feedchannels 503 are filled with a spacer that keeps the sheetssubstantially evenly spaced apart.

The heat capacity of the metal foils 509 limits the thermal excursionsin the process. When CO₂ is adsorbed in the adsorbent, heat is releasedin the amount of the heat of adsorption. This warms the adsorbent filmsand as the film warms, its temperature rises above that of the metalfoils and heat diffuses into the metal foil where it is stored.Desorption of CO₂ from the adsorbent is an endothermic process and heatmust be supplied in an amount equal to the heat of adsorption. When CO₂desorbs, the temperature of the films falls below that of the metalfoils and heat stored in the metal foils flows into the films. Thethermal excursion of the adsorbent film is less than +/−10° C. with thecontactor dimensions and the process as described in Example 1.

The adsorbent film is composed of individual adsorbent crystals,mesopores (including grain boundaries) and macropores. In this example,the crystals in the film are substantially of the same size. Most of theopen volume in the film is comprised of mesoporous cracks withcharacteristic widths of about 200 angstroms. These mesoporous cracksare substantially evenly distributed throughout the film. The totalvolume of the mesopores and macropores is about 5 vol. % of the totalvolume of the adsorbent film.

The present invention can better be understood with reference to thefollowing examples that are presented for illustrative purposes and notto be taken as limiting the invention.

EXAMPLE 1

With a laminated sheet parallel channel contactor described for FIG. 11hereof, a PSA/RCPSA cycle with five steps is operated to produce aproduct stream containing about 20 vol. % CO₂ and about 80 vol. % CH₄.Overall methane recovery for the PSA/RCPSA cycle is computed to be about95 vol. %. FIG. 12 hereof is a schematic diagram of five different stepsin a preferred PSA/RCPSA cycle suitable for use in this invention. Inthe first step 611 a parallel channel contactor PSA/RCPSA cycle ispressurized with high pressure product gas 687. This pressurizationraises the pressure in the parallel channel contactor and fills thecontactor with the purified product containing about 20 vol. % CO₂ andabout 80 vol. % CH₄. In a second step 621 a high pressure 51 atmosphere(atm) feed gas 671 is conducted through the parallel channel contactor.During this step 621 the DDR adsorbent layer adsorbs CO₂ from theflowing feed gas 671. A purified product 625 flows out of the end of thecontactor. The feed gas 671 is flowed at a rate such that as the product625 emerges from the parallel channel contactor as a concentration frontmoves through the contactor. Ahead of the front the gas has acomposition near that of the product 625. Behind the front the gas has acomposition near that of the feed 671. The second step 621 is stoppedbefore this front completely breaks-through the end of the contactor.The amount of feed which emerges from the contactor before this step ishalted determines in part the product purity.

At this point, a third step of the cycle 631 is initiated which servesto purge the contactor of feed gas trapped in the contactor channels.The third step 631 also acts, in part, as a partial pressuredisplacement purge of the contactor. Valves are opened at the top andbottom of the contactor. A pressurized CO₂ rich stream 633 flows intothe top of the module and gas originally contained in the flow channel503 of the structured parallel channel contactor flows out 639. The gasfed into the top of the module 633 is a CO₂-rich gas produced in latersteps 4 and 5 that has been compressed 675 to a pressure slightlygreater than the feed pressure (51 atm.). The composition of the gas fedthrough the top of the contactor is substantially equal to that of theCO₂ reject stream 681, containing 97.5 vol. % CO₂ and 2.5 vol. % CH₄.The gas exiting out the bottom of the contactor has a composition nearerto that of the feed gas 671 (70 vol. % CO₂ and 30 vol. % CH₄).

As the gas stream entering the module 633 displaces the gas in the flowchannels, a compositional front moves from top to bottom of the module.The third step 631 is stopped and a fourth step 641 is begun before, orshortly after, this front breaks through the bottom of the module. Thefourth step 641 lets the pressure of the contactor down to anintermediate pressure and recovers some of the CO₂ for recompression. Inthe design discussed in this example, the intermediate pressure is about22 atm. In the fourth step, a CO₂-rich stream 649 exits the module at apressure of about 22 atm. This stream is split into two streams 679 and681. Stream 679 is fed to compressor 675 and stream 681 is rejected fromthe process at a pressure of about 22 atm. Stream 633 that was used torinse the contactor in the third step of the process 631, is comprisedof the gas stream 679 that emerges from compressor 675. As the pressurein the contactor drops towards the outlet pressure of about 22 atm., theflow in streams 679 and 681 decreases. When the flow in these streamshas fallen to approximately ¼ of the initial value step 4 is stopped anda step 5 is begun. In the fifth step of the process 651, the modulepressure is dropped to about 5 atm. and a CO₂-rich stream is recovered685. This stream 685 can optionally be fed through a compressor 677 thatraises the stream pressure to about 22 atm. The stream is then combinedwith stream 681 and a CO₂-rich stream 683 is rejected from the processat a pressure of about 22 atmospheres.

To improve the operation of the process, as well as the pressure atwhich CO₂ is recovered, gas may be recovered in the fifth step 651 usinga multi-step process in which the contactor pressure is decreased in aseries of pressure equalization steps. Gas from these pressureequalization steps can be recovered as individual gas streams andrecompressed. In an example with two pressure equalization steps, oneportion of the CO₂-rich gas is recovered at a pressure of about 12atmospheres while the rest is recovered at about 5 atmospheres.

It is also possible to decrease the module pressure in step four 641using a series of pressure equalization steps. Again, each pressureequalization step can be used to form a gas stream that can either berejected from the process in stream 683 or recompressed to form stream633. If pressure equalization steps are employed, it is advantageous todesign them to maximize the pressure at which the CO₂ reject streams arecaptured.

Modeling used to predict the performance of the parallel channelcontactor uses isotherms for CO₂ and CH₄ that were measured with wellknown gravimetric uptake methods, PVT (pressure, volume, temperature)methods, and with analysis of single component gas transport data in DDRmembranes. A statistical isotherm shape was found to best describe thesingle component isotherms for CO₂ and CH₄. The best fits to themeasured isotherms for CO₂ and CH₄ give saturation capacities of 6 and 5molecules per cage, respectively, in the DDR zeolite framework. Thesevalues correspond to a maximum loading of 5 milli-moles/gram (of DDR)for CO₂ and 4.16 milli-moles/gram (of DDR) for CH₄. These saturationcapacities are consistent with the physical expectation that the maximumpossible loading would correspond to CO₂ and CH₄ filling the pores at aliquid density. A single parameter K, that is analogous to the Henry'sconstant in a Langmuir isotherm, describes the shape of the statisticalisotherm. The K values used for modeling are:

$K_{{CO}_{2}} = {1.93 \times 10^{- 10}{\mathbb{e}}^{\frac{25 \times 10^{3}\frac{Joule}{Mole}}{RT}}\mspace{31mu}\left( {{in}\mspace{14mu}{pascals}^{- 1}} \right)}$$K_{{CH}_{4}} = {4.25 \times 10^{- 10}{\mathbb{e}}^{\frac{17.8 \times 10^{3}\frac{Joule}{Mole}}{RT}}\mspace{14mu}\left( {{in}\mspace{14mu}{pascals}^{- 1}} \right)}$where R is the molar gas constant and T is the temperature in Kelvin.

Over a wide range of conditions (less than about 50% of saturationcapacity loading), the shape of the statistical and Langmuir isothermsare very similar. For simple modeling of the process given in thisexample, the statistical isotherm can be supplanted by an equivalentLangmuir isotherm. For modeling competitive adsorption effects,competitive adsorption isotherms can be derived from the singlecomponent statistical isotherms using well known techniques.

The single component Stefan-Maxwell transport diffusion coefficients forCO₂ and CH₄ used in the modeling hereof were:

$D_{{CO}_{2}} = {5.70 \times 10^{- 10}{\mathbb{e}}^{- \frac{7.4` \times 10^{3}\frac{Joule}{Mole}}{RT}}\mspace{20mu}\left( {{in}\mspace{14mu} m^{2}\text{/}\sec} \right)}$$D_{{CH}_{4}} = {0.48 \times 10^{- 10}{\mathbb{e}}^{- \frac{13.4 \times 10^{3}\frac{Joule}{Mole}}{RT}}\mspace{11mu}\left( {{in}\mspace{14mu} m^{2}\text{/}\sec} \right)}$where R is the molar gas constant and T is the temperature in Kelvin.

It is seen that there is a large difference in the diffusioncoefficients of CO₂ and CH₄ and a smaller difference in the isothermswhen these transport parameters are evaluated at a given temperature.From a 50/50 molar mixture of CO₂ and CH₄ the isotherms slightly favorCO₂ adsorption and dramatically favor the diffusional transport of CO₂into the DDR crystals. By controlling the time scale of the adsorptionstep 621 and the purge displacement step 631, it is possible to takeadvantage of this difference in diffusion coefficients and improve theselectivity of the process. By controlling these time steps, a kineticseparation of CO₂ and CH₄ can be achieved that takes advantage ofdifferences in diffusivity of these molecules. The class of 8-ringzeolites preferred for the removal of CO₂ from natural gas will have alarge difference in CO₂ and CH₄ diffusion coefficients. This exampleillustrates a particular RCPSA cycle that can be tuned to achieve akinetic separation of CO₂ and CH₄, however, other swing adsorptioncycles are possible. A parameter that can be used to evaluate theability of a given material to produce a kinetic separation is the ratioof diffusion coefficients for the components that are to be separatedand the diffusion coefficients are evaluated at the temperature andpressure of the intended process,

$\kappa = \frac{D_{{CO}_{2}}}{D_{{CH}_{4}}}$

It is preferred that the material be chosen to have a value of κ for CO₂and methane separation greater than 10 at the operating temperature.More preferably the material is chosen to have a value of κ greater than25 at the operating temperature. More preferably the material is chosento have a value of κ greater than 50 at the operating temperature.

In order to take advantage of the intrinsic kinetic selectivity of thepreferred class of 8-ring zeolite materials for removal of CO₂ fromnatural gas, the crystals forming the contactor must be of substantiallythe same size. If they have widely different sizes, some willsubstantially fill with CH₄ during the adsorption step 621, resulting inincreased methane loss during the desorption step 651. It is thereforepreferred that the standard deviation of the volume of the individualcrystallites in the DDR film forming the adsorbent layer 505 (as seen inFIG. 11) be less than 100% of the volume of an average crystallite inorder to increase methane recovery in the process. In a more preferredembodiment, the standard deviation of the volume of the crystallites inthe DDR film forming the adsorbent layer 505 is less than 50% of thevolume of an average crystallite. In the most preferred embodiment, thestandard deviation of the volume of the crystallites in the DDR filmforming the adsorbent layer 505 is less than 10% of the volume of anaverage crystallite. The most preferred embodiment was chosen to modelthe PSA cycle described in this example.

With this type of adsorbent, the time for steps 621 and 631 is set bythe average crystal size in the adsorbent. It is preferred that the timestep be chosen so that adsorbed CO₂ in the DDR has time to equilibratewith gaseous CO₂ in the feed channel 503 but the methane does not havetime to equilibrate. The time for CO₂ to achieve 90% approach toequilibrium (following a change in surface concentration) within asingle DDR crystal that has rapid diffusion path to the gas in thecontactor is;τ₉₀=0.183r ² /D _(CO2)where r is the average DDR crystal radius and D_(CO2) is the diffusioncoefficient of CO₂ at the operating temperature. It is preferred thatthe time for steps 621 and 631 be in a range from 0.5 τ₉₀ to 10 τ₉₀ andit is more preferred that the time for steps 621 and 631 be in a rangefrom 1 τ₉₀ to 5 τ₉₀. For modeling, a time step of 1.5 τ₉₀ was chosen forsteps 621 and 631. The numerical value of the time step is then set bythe crystal size. It is preferred that the average DDR crystal size bein a range from about 0.005 μm to about 100 μm. It is more preferredthat the average DDR crystal size be in a range from about 0.5 μm toabout 50 μm and it is most preferred that the average DDR crystal sizebe in a range from about 1 μm to about 10 μm. For modeling an averageDDR crystal size of 1 μm was used.

Several different treatments of the molecular transport into and out ofthe adsorbent layer were developed and results of the different modelingapproximations were compared. The most exact treatment solved the timedependent fundamental multi-component transport equations into and outof the DDR zeolite layer at all points along the feed channel for everytime step in the process. For this model the DDR film was idealized asfins along the side of the channel with 200 angstrom gaps between thefins. The mesopores formed by these 200 angstrom gaps occupied 5% of thevolume of the adsorbent layer. On a grid encompassing 500,000 points,the fundamental time dependent transport equations were solved for thisgeometry. Three pressure equalization steps were used in the blow-downstep 651 for process modeling. Pressure equalizations at 15, 10 and 5atmospheres were employed. It was determined that the thermal excursionin the adsorption step was 10° C. The methane recovery was computed asthe ratio of the methane molar flow rate in purified product stream 625to that in the feed stream 671 entering the process. The model using themost exact treatment of transport showed a 96% methane recovery. Theaverage pressure at which molecules were recovered in streams 681 and685 was found to be 12.5 atmospheres.

This modeling approach provides a much more exact solution than thelinear driving force (LDF) models that are conventionally used to modelPSA processes. Using knowledge of the most exact solution, a simplermodel has been constructed that can readily be used by one skilled inthe art to compute methane recoveries. In the adsorption step 621, thesimple model separately treats the equilibration of CO₂ and CH₄ in thefeed channel within the DDR crystals forming the adsorbent layer. Theamount of CO₂ adsorbed in the DDR crystals is taken to be 80% of theamount that might be expected if the CO₂ adsorbed in the DDR were fullyequilibrated with the gaseous feed entering the process 671 at atemperature that is 10° C. higher than the feed temperature. The amountof CH₄ adsorbed in the DDR crystals is 1% of the amount that might beexpected if the CH₄ adsorbed in the DDR has fully equilibrated with thegaseous feed entering the process 671 at a temperature that is 10° C.higher than the feed temperature. In the simple model gas filling themesopores and macropores in the laminate at the end of the adsorptionstep 621 is not recovered. For the simple model, the CO₂ purge used instep 631 displaces all of the methane left in the feed channel intostream 639. With these approximations, the methane recovery from theprocess is predicted to be 95%. This closely agrees with the exact modeland the model provides a simple method for one skilled in the art toevaluate the effect of changing the meso and macropore volume of theadsorbent.

Optionally when the CO₂ reject stream 683 is sequestered it is preferredto capture the CO₂ at a pressure that is more than 1/10 of the partialCO₂ pressure in the feed. In a more preferred embodiment the pressure atwhich the CO₂ is captured is more than ¼ of the CO₂ partial pressure inthe feed.

EXAMPLE 2

The process of Example 1 is repeated except the feed and productspecifications are held constant and the volume fraction of meso andmacropores in the adsorbent are increased from 5% to 10%. The predictedmethane recovery is found to fall from 95% (in Example 1) to 92%.

EXAMPLE 3

The process of Example 1 is repeated except the feed and productspecifications are held constant and the volume fraction of meso andmacropores in the adsorbent are increased from 5% to 15%. The predictedmethane recovery is found to fall from 95% (in Example 1) to 88%.

EXAMPLE 4

The process of Example 1 is repeated except the feed and productspecifications are held constant and the volume fraction of meso andmacropores in the adsorbent are increased from 5% to 20%. The predictedmethane recovery is found to fall from 95% (in Example 1) to 85%.

EXAMPLE 5

The process of Example 1 is repeated except the feed and productspecifications are held constant and the volume fraction of meso andmacropores in the adsorbent are increased from 5% to 25%. The predictedmethane recovery is found to fall from 95% (in Example 1) to 81%.

EXAMPLE 6

The process of Example 1 is repeated except the feed and productspecifications are held constant and the volume fraction of meso andmacropores in the adsorbent are increased from 5% to 30%. The predictedmethane recovery is found to fall from 95% (in Example 1) to 77%.

EXAMPLE 7

The process of Example 1 is repeated except the feed and productspecifications are held constant and the volume fraction of meso andmacropores in the adsorbent are increased from 5% to 35%. The predictedmethane recovery is found to fall from 95% (in Example 1) to 73%.

EXAMPLE 8

The process of Example 1 is repeated except the feed and productspecifications are held constant and the volume fraction of meso andmacropores in the adsorbent are decreased from 5% to 2.5%. The predictedmethane recovery is found to increase from 95% (in Example 1) to 97%.

EXAMPLE 9

The process of Example 1 is repeated but the feed specification is heldconstant and purity of methane in the product stream 625 is increasedfrom 80% CH₄/20% CO₂ to 90% CH₄/10% CO₂. This is done by changing thefeed flow rates and the degree to which compositional fronts are allowedto break-through before steps 621 and 631 are stopped. With a 5% volumefraction of meso and macropores in the adsorbent the predicted methanerecovery is 94%.

EXAMPLE 10

The process of Example 1 is followed but the feed specification is heldconstant and purity of methane in the product stream 625 is increasedfrom 80% CH₄/20% CO₂ to 90% CH₄/10% CO₂. This is done by changing thefeed flow rates and the degree to which compositional fronts are allowedto break-through before steps 621 and 631 are stopped. With a 10% volumefraction of meso and macropores in the adsorbent the predicted methanerecovery is 90%.

EXAMPLE 11

This example illustrates use of a parallel contactor of the presentinvention in a separation process that produces relatively high pressureproducts and high methane recoveries from N₂ containing natural gasstream. In processing natural gas, the amount of N₂ that has to beremoved depends on the concentration in the field and the way in whichthe gas is transported to market (i.e., liquefied natural gas vs.pipeline). This example will consider a natural gas stream containing asmall amount (<5%) of impurities (for example H₂O and mercury compounds)other than N₂. These impurities are removed in initial processing stepsusing conventional separation techniques. The gas stream fed to theparallel contactor of the present invention has a composition of 70% CH₄and 30% N₂. The flowing gas stream is fed to the contactor at a pressureof 100 atmospheres and a temperature of 50° C.

The contactor is comprised of laminated flat sheets of the typedescribed above and a schematic diagram of the type of sheet used in thepresent example is shown in FIG. 11 hereof. Using the methods describedabove a 50 μm thick DDR film 505 with an Si/Al ratio greater than 100 ishydrothermally grown on each of both faces of a 100 μm thick flat metalfoil 509 (for example stainless steel). The metal foils 509 with the DDRfilms 505 are laminated together 501 to form a parallel channelcontactor. During lamination, spacers are placed between the metal foilsso that the channel gap 503 is 50 μm across. Approximately half thevolume of the feed channels 503 are filled with spacers that keep thesheets substantially evenly spaced 50 μm apart.

The heat capacity of the metal foil 509 limits the thermal excursions inthe process. When N₂ is adsorbed in DDR it releases heat in the amountof the heat of adsorption. This warms the DDR film. The DDR film warmsto a temperature above that of the metal foil and heat diffuses into themetal foil where it is stored. Desorption of N₂ from DDR is anendothermic process and heat must be supplied in the amount of the heatof the adsorption. When N₂ desorbs the temperature of the DDR film fallsbelow that of the metal foil and heat stored in the foil flows into theDDR film. With the contactor dimensions and the process described inthis example, the thermal excursion of the DDR film is expected to beless than +/−5° C. Due to the smaller heat of adsorption for N₂(compared to CO₂) this temperature rise is less than that of thedescription for Example 1 above.

The DDR film is comprised of individual DDR crystals, mesopores 1 5(including grain boundaries) and macropores. In this example, thecrystals in the DDR film are substantially of the same size. Most of theopen volume in the film is comprised of mesoporous cracks withcharacteristic widths of 200 angstroms. These mesoporous cracks aresubstantially evenly distributed throughout the film. The total volumeof the meso and macropores is 2.5% of the total volume of the in the DDRfilm.

Using this parallel channel contactor, a PSA/RCPSA cycle with fivedifferent steps is operated to produce product stream containing 2% N₂and 98% CH₄. Overall methane recovery for the PSA/RCPSA cycle iscomputed to be 91%. FIG. 13 hereof shows a schematic diagram of the fivedifferent steps in the PSA/RCPSA cycle. In the first step 711 a parallelchannel contactor PSA/RCPSA cycle is pressurized with high pressureproduct gas 787. This pressurization raises the pressure in the parallelchannel contactor and fills the contactor with the purified productcontaining 2% N₂ and 98% CH₄. In a second step 721 a high pressure 100atm feed gas 771 is flowed through the contactor. During this step 721the DDR adsorbent layer removes N₂ from the flowing feed gas 771. Apurified product 725 flows out of the end of the contactor. The feed gas771 is flowed at a rate such that as the product 725 emerges from thecontactor a concentration front moves through the contactor. Ahead ofthe front the gas has a composition near that of the product 725. Behindthe front the gas has a composition near that of the feed 771. Beforethis front completely breaks through the end of the contactor the secondstep 721 is stopped. The amount of feed which emerges from the contactorbefore this step is halted determines in part the product purity.

At this point, a third step of the cycle 731 is initiated which servesto purge the contactor of feed gas trapped in the contactor channels.The third step 731 also acts in part as a partial pressure displacementpurge of the contactor. Valves are opened at the top and bottom of thecontactor. A pressurized N₂ rich stream 733 flows into the top of themodule and gas originally contained in the flow channel 503 (of FIG. 11hereof) of the structured parallel channel contactor flows out 739. Thegas fed into the top of the module 733 is a N₂ rich gas produced inlater steps (4 and 5) that has been compressed 775 to a pressureslightly greater than the feed pressure (100 atm.). Composition of thegas fed in through the top of the contactor is nearly that of the N₂reject stream 781. The gas exiting out the bottom of the contactor has acomposition nearer to that of the feed gas 771 (30% N₂ and 70% CH₄).

As the gas stream entering the module 733 displaces the gas in the feedchannel a compositional front moves from top to bottom of the module.Before or shortly after this front breaks through the bottom of themodule the third step 731 is stopped and a fourth step 741 is begun. Thefourth step 741 lets the pressure of the contactor down to anintermediate pressure and recovers some of the N₂ for recompression. Inthe design discussed in this example the intermediate pressure is 30atm. In the fourth step a N₂ rich stream 749 exits the module at apressure of 30 atm. This stream is split into two streams 779 and 781.Gas in stream 779 is fed to a compressor 775 and gas in stream 781 isrejected from the process at a pressure of 30 atm. In an optimization ofthis process a pressure in step 741 is chosen that minimizes the amountof gas flowing in stream 781. Stream 733 that was used to rinse thecontactor in the third step of the process 731 is comprised of the gasstream 779 that emerged from the compressor 775. As the pressure in thecontactor drops towards the outlet pressure of 30 atm, the flow instreams 779 and 781 decrease. When the flow in these streams has fallento approximately ¼ of the initial value the fourth step is stopped and afifth step is begun. In the fifth step of the process 751 the modulepressure is dropped to 1.2 atm and a N₂ rich stream is recovered 785.

To improve the operation of the process, as well as the pressure atwhich N₂ is recovered, gas may be recovered in the fifth step 751 usinga multi-step process in which the contactor pressure is decreased in aseries of pressure equalization steps. In an example with two pressureequalization steps, one portion of the N₂ rich gas is recovered at apressure of 12 atmospheres while the rest is recovered at 1.2atmospheres.

It is also possible to decrease the module pressure in step four 741with a series of pressure equalization steps. Again each pressureequalization step can be used to form a gas stream that can either berejected from the process in stream 783 or recompressed to form stream733. If pressure equalization steps are employed is it advantageous todesign them to maximize the pressure at which the N₂ reject streams arecaptured.

The modeling used to predict the performance of the contactor usedisotherms for N₂ and CH₄ that were measured with well known gravimetricuptake methods, PVT (pressure, volume, temperature) methods, and withanalysis of single component transport data in DDR membranes. Astatistical isotherm shape was found to best describe the singlecomponent isotherms for N₂ and CH₄. The best fits to the measuredisotherms for N₂ and CH₄ give saturation capacities of 5 molecules parcage in the DDR zeolite framework. These values correspond to a maximumloading of 4.17 milli-moles/gram (of DDR). These saturation capacitiesare consistent with the physical expectation that the maximum possibleloading would correspond to N₂ and CH₄ filling the pores at a liquiddensity. A single parameter K, that is analogous to the Henry's constantin a Langmuir isotherm, describes the shape of the statistical isotherm.The K values used for modeling are:

$K_{N_{2}} = {3.79 \times 10^{- 9}{\mathbb{e}}^{\frac{9.6 \times 10^{3}\frac{Joule}{Mole}}{RT}}\mspace{45mu}\left( {{in}\mspace{14mu}{pascals}^{- 1}} \right)}$$K_{{CH}_{4}} = {4.25 \times 10^{- 10}{\mathbb{e}}^{\frac{17.8 \times 10^{3}\frac{Joule}{Mole}}{RT}}\mspace{14mu}\left( {{in}\mspace{14mu}{pascals}^{- 1}} \right)}$where R is the molar gas constant and T is the temperature in Kelvin.

Over a wide range of conditions the shape of the statistical andLangmuir isotherms are very similar. For simple modeling of the processgiven in this example, the statistical isotherm can be supplanted by anequivalent Langmuir isotherm. For modeling competitive adsorptioneffects, competitive adsorption isotherms can be derived from the singlecomponent statistical isotherms using well known techniques.

The single component Stefan-Maxwell transport diffusion coefficients forN₂ and CH₄ used in the modeling were

$D_{N_{2}} = {0.48 \times 10^{- 10}{\mathbb{e}}^{- \frac{1.5 \times 10^{3}\frac{Joule}{Mole}}{RT}}\mspace{40mu}\left( {{in}\mspace{14mu} m^{2}\text{/}\sec} \right)}$$D_{{CH}_{4}} = {0.48 \times 10^{- 10}{\mathbb{e}}^{- \frac{13.4 \times 10^{3}\frac{Joule}{Mole}}{RT}}\mspace{14mu}\left( {{in}\mspace{14mu} m^{2}\text{/}\sec} \right)}$where R is the molar gas constant and T is the temperature in Kelvin.

When these transport parameters are evaluated at a given temperature, itis seen that there is a large difference in the diffusion coefficientsof N₂ and CH₄ and a smaller difference in the isotherms. From a 50/50molar mixture of N₂ and CH₄, the isotherms slightly favor CH₄ adsorptionbut dramatically favor the diffusional transport of N₂ into the DDRcrystals. By controlling the time scale of the adsorption step 721 andthe purge displacement step 731, it is possible to take advantage ofthis difference in diffusion coefficients and improve the selectivity ofthe process. By controlling these time steps a kinetic separation of N₂and CH₄ can be achieved that takes advantage of differences indiffusivity of these molecules. The preferred class of 8-ring zeolitematerials for removal of N₂ from natural gas will have a largedifference in N₂ and CH₄ diffusion coefficients. This exampleillustrates a particular RCPSA cycle that can be tuned to achieve akinetic separation of N₂ and CH₄; however, other swing adsorption cyclesare possible. A parameter that can be used to evaluate the ability of agiven material to produce a kinetic separation is the ratio of thesingle component diffusion coefficients for the components. The ratio isevaluated at the temperature of the intended process,

$\kappa = \frac{D_{N_{2}}}{D_{{CH}_{4}}}$Values of κ for DDR for nitrogen and methane separation at severaldifferent temperatures are given in the table below:

Temperature (C.) κ 20 130 40 100 60 75 80 60 100 50

For the preferred class of 8-ring zeolite materials for removal of N₂from natural gas κ_(N2/CH4) is a function of temperature. It ispreferred that the material be chosen to have a value of κ_(N2/CH4)greater than 5 at the operating temperature. More preferably thematerial is chosen to have a value of κ_(N2/CH4) greater than 20 at theoperating temperature. Even more preferably the material is chosen tohave a value of κ_(N2/CH4) greater than 50 at the operating temperature.

In order to take advantage of the intrinsic kinetic selectivity of thepreferred class of 8-ring zeolite materials for removal of N₂ fromnatural gas, the crystals forming the contactor must have substantiallythe same size. If they have widely different sizes some willsubstantially fill with CH₄ during the adsorption step 721, resulting inincreased methane loss during the desorption step 751. To increasemethane recovery in the process it is then preferred that the standarddeviation of the volume of the individual crystallites in the DDR filmforming the adsorbent layer 505 be less than 100% of the volume of anaverage crystallite. In a more preferred embodiment the standarddeviation of the volume of the crystallites in the DDR film forming theadsorbent layer 505 is less than 50% of the volume of an averagecrystallite. In the most preferred embodiment the standard deviation ofthe volume of the crystallites in the DDR film forming the adsorbentlayer 505 is less than 10% of the volume of an average crystallite. Themost preferred embodiment was chosen to model the PSA cycle described inthis example.

With this type of adsorbent, the time for steps 721 and 731 is set bythe average crystal size in the adsorbent. It is preferred that the timestep be chosen so that adsorbed N₂ in the DDR has time to equilibratewith gaseous N₂ in the feed channel 503 but the CH₄ does not have timeto equilibrate. The time for N₂ or CH₄ achieve 90% approach toequilibrium (following a change in surface concentration) within asingle DDR crystal that has rapid diffusion path to the gas in thecontactor is;τ₉₀=0.183r ² /D _(N2)andτ₉₀=0.183r ² /D _(CH4)where r is the average DDR crystal radius and D_(N2) and D_(CH4) are thediffusion coefficients of N₂ and CH₄ at the operating temperature. Ifthere are no external mass transfer limitations, the equilibration timesfor different crystallite sizes are given in the table below:

Crystallite Size (μm) Nitrogen τ₉₀ (seconds) Methane τ₉₀ (seconds) 1.50.01 2 5 0.1 22 10 0.45 91 25 2.8 570 40 7.3 1460

It is preferred that for N₂ the time for steps 721 and 731 be in a rangefrom 0.5 τ₉₀ to 10 τ₉₀ and it is more preferred that the time for steps721 and 731 be in a range from 1 τ₉₀ to 5 τ₉₀. For modeling, a time stepof 1.5 τ₉₀ was chosen for steps 721 and 731. The numerical value of thetime step is then set by the crystal size. It is preferred that theaverage DDR crystal size be in a range from 0.005 μm to 100 μm. It ismore preferred that the average DDR crystal size be in a range from 0.5μm to 50 μm and it is most preferred that the average DDR crystal sizebe in a range from 1 μm to 10 μm. For modeling an average DDR crystalsize of 5 μm was used.

A simplified modeling approach similar to that described above was used.In the adsorption step 721, the simple model separately treats theequilibration of N₂ and CH₄ in the feed channel with the DDR crystalsforming the adsorbent layer. The amount of N₂ adsorbed in the DDRcrystals is taken to be 80% of the amount that might be expected if theN₂ adsorbed in the DDR has fully equilibrated with the gaseous feedentering the process 771 at a temperature that is 10° C. higher than thefeed temperature. The amount of CH₄ adsorbed in the DDR crystals is 1%of the amount that might be expected if the CH₄ adsorbed in the DDR hasfully equilibrated with the gaseous feed entering the process 771 at atemperature that is 10° C. higher than the feed temperature. In thesimple model, gas filling the mesopores and macropores in the laminateat the end of the adsorption step 721 is not recovered. Also, for thesimple model the N₂ purge used in step 731 displaces all of the methaneleft in the feed channel into stream 739. With these approximations themethane recovery from the process is predicted to be 91%. This closelyagrees with the exact model and the model provides a simple method forone skilled in the art to evaluate the effect of changing the meso andmacropore volume of the adsorbent.

EXAMPLE 12

The process of Example 11 is followed but the feed and productspecifications are held constant and the volume fraction of meso andmacropores in the adsorbent are increased from 2.5% to 5%. The predictedmethane recovery is found to fall from 91% (in Example 11) to 90%.

EXAMPLE 13

The process of Example 11 is followed but the feed and productspecifications are held constant and the volume fraction of meso andmacropores in the adsorbent are increased from 2.5% to 15%. Thepredicted methane recovery is found to fall from 91% (in Example 11) to84%.

EXAMPLE 14

The process of Example 11 is followed but the feed and productspecifications are held constant and the volume fraction of meso andmacropores in the adsorbent are increased from 2.5% to 20%. Thepredicted methane recovery is found to fall from 91% (in Example 11) to81%.

EXAMPLE 15

The process of Example 11 is followed but the feed and productspecifications are held constant and the volume fraction of meso andmacropores in the adsorbent are increased from 2.5% to 25%. Thepredicted methane recovery is found to fall from 91% (in Example 11) to78%.

EXAMPLE 16

This example illustrates the use of a turboexpander to condition sourgas (i.e., natural gas-containing H₂S and CO₂) so that PSA can operatein the window that optimizes methane recovery. FIG. 14 hereof shows aprocess scheme in which a turboexpander is used to set the pressure andtemperature of a sour gas that is separated in a PSA apparatus. A sourgas stream 811 with a temperature of 100° C. and a pressure of 1,500 psiis produced from a gas field and fed to the process. The CO₂ content ofthe stream is 66 mole % and the H₂S concentration is 2 mole %. Water ispresent at its saturated vapor pressure, and the concentration of theheavy hydrocarbons is 2 mole %. The heavy hydrocarbons contain a smallfraction of waxy species with carbon numbers as large as 36. For thisstream 811, CO₂ comprises the majority of the heavy component that willbe removed by a kinetically controlled PSA process. If DDR zeolite isused as the adsorbent in the kinetically controlled PSA, the loading inthe DDR zeolite from CO₂ partial pressure in stream 811 would be inexcess of 0.6 q_(s) and the slope of the CO₂ isotherm would be:

$\frac{\partial q_{{CO}\; 2}}{\partial P_{{CO}\; 2}} \cong {{.02}\mspace{11mu} K_{{CO}\; 2}\mspace{11mu} q_{s}}$where K_(CO2) is the Henry's constant for CO₂, and q_(s) is thesaturated loading for CO₂ in DDR.

To bring the stream into a more preferred window of operation the streamis passed through a turboexpander 821 that reduces the stream pressureto 500 psi. In a preferred embodiment, the turboexpander 821 is designedto have a radial inflow. Radial inflow turbine designs suitable for usein this process can be found in Perry's Chemical Engineers' Handbook(7th Edition. ©1997 McGraw-Hill edited by R. H. Perry and D. W. Green).During the approximately isentropic expansion, the gas temperature fallssignificantly and liquids may fall out of the gas stream due to a changein the dew point and reduction in temperature. Radial inflow turbinedesigns can be operated so that liquids falling out of the gas streamwill not impede the operation of the turboexpander. In this example, thepower generated by the turboexpander is coupled through a shaft 831 toand an electric generator 823. In an alternative embodiment, the powerfrom the turboexpander shaft is coupled to a compressor instead of anelectric generator.

Before the stream is passed through turboexpander 821, it may optionallybe sent through a process 813 to remove any particles, or a portion ofthe wax, or optionally some of the heavy hydrocarbons, H₂S and/or water.The absolute temperature of the stream 837 coming out of theturboexpander is approximately 30% less than feedstream 811, and itcontains a mixture of gas and liquid droplets. Stream 837 is then sentto a process block 839 that at least removes the liquid droplets fromthe stream. Liquid droplet removal can be accomplished through a varietyof methods including coalescing filters, settling drums, staticcentrifugation, and electrostatic precipitation. The process block 839also contains equipment to increase the temperature of stream 837. Meansof heating the stream within process block 839 include heat exchangerssuch as shell and tube heat exchangers as well as other types of heatexchangers including the many varieties discussed in Perry's ChemicalEngineers' Handbook (7th Edition. ©1997 McGraw-Hill edited by R. H.Perry and D. W. Green), including packed bed heat exchangers.Alternatively, stream 837 may be by mixing it with a separately formedhot gas stream 835. When heat exchangers are used in process block 839,it is preferred that they extract heat from stream 881, 891, 895, orsome combination thereof using a multi-pass heat exchanger. Optionally,the heat exchanger used in process block 839 can extract heat from anoptional stream 835. In one embodiment stream 835 is produced bycombusting hydrocarbon and air or oxygen enriched air. In anotherembodiment, stream 835 is produced by heat exchanging a working fluid orgas with high temperature combustion products. Besides increasing thetemperature of stream 837 and removing liquid droplets, process block839 can optionally be configured to remove heavy hydrocarbons, watervapor, or H₂S from the gas phase. In this example, process block 839 isconfigured to remove liquid droplets, and to heat stream 837 to atemperature of 90° C.

The physical composition of stream 871 coming from process block 839 issuch that if DDR zeolite is used as the adsorbent in a kineticallycontrolled PSA, the loading in the DDR zeolite from CO₂ partial pressurein stream 871 would be in excess of 0.5 q_(s) and the slope of the CO₂isotherm would be:

$\frac{\partial q_{{CO}\; 2}}{\partial P_{{CO}\; 2}} \cong {{.07}\mspace{11mu} K_{{CO}\; 2}\; q_{s}}$where K_(CO2) is the Henry's constant for CO₂ at 90° C. and q_(s) is thesaturated loading for CO₂ in DDR.

This operating condition is in a more desirable range for high methanerecovery with a kinetically controlled PSA process than that for stream811. PSA unit 841 is used to separate most of the CO₂ and a fraction ofthe H₂S out of stream 871. In a preferred embodiment, PSA unit 841contains a parallel channel contactor with an adsorbent having an openvolume fraction of mesopores and macropores that is less than 10%. In apreferred embodiment, the microporous adsorbent in the contactor is an8-ring zeolite and PSA unit 841 is a RCPSA unit that is operated in akinetically controlled mode. In a preferred embodiment more than 90% ofthe methane and heavy hydrocarbon fed to PSA unit 841 is recovered inthe methane enriched stream 815. In a an even more preferred embodiment,more than 95% of the methane and heavy hydrocarbon fed to PSA unit 841is recovered in the methane enriched stream 815. In this example themolar ratio of methane to CO₂ in the methane enriched stream 815 isgreater than 9:1. Depending upon final use, the methane enriched stream815 may be further processed or purified in other processes. The CO₂enriched stream 881 coming from the PSA 841 can be sent through anoptional process block 851 to remove water vapor.

The optional process block 851 can also contain one side of a heatexchanger that is used to provide heat to the heat exchanger in processblock 839. The CO₂ in stream 881 is ultimately sent to a compressor 829.The compressor 829 is preferably driven by the energy recovered from theturboexpander 821. In this example energy produced by the electricgenerator 823 is sent through a power transmission line 825 to power amotor 827 that is shaft-coupled via 833 to the compressor 829. As waspreviously mentioned in an alternative embodiment, the compressor 829can be shaft-coupled to the turboexpander 821. Because of the work ofcompression, the temperature of the stream 891 coming out of thecompressor 829 is greater than that of stream 881. It can beadvantageous to cool this stream 891 before further compression topressures required for CO₂ disposal/sequestration. Cooling can beaccomplished with an optional process block 883 that contains one sideof a heat exchanger that is used to provide heat to the heat exchangerin process block 839. If needed, process block 883 can contain equipmentsuch as a glycol dehydration unit to reduce the corrosivity of the gasmixture. To raise the pressure of the CO₂ rich gas stream 893 to thelevel needed for CO₂ disposal/sequestration, a final compressor 897 isprovided. The compressed CO₂ rich gas stream 895 is injected into anunderground formation for CO₂ disposal/sequestration, or for enhancedoil recovery.

What is claimed is:
 1. A process for removing a target gas componentfrom a gas mixture containing said target gas component and a second gascomponent, which process comprises: a) conducting said gas mixture to aswing adsorption gas separation unit wherein the gas separation unitcontains at least one adsorbent contactor comprising a gas inlet and agas outlet, wherein the gas inlet and the gas outlet are in fluidconnection by a plurality of open flow channels wherein the surfaces ofthe open flow channels are comprised of an adsorbent material that has aselectivity for said target gas component over said second gas componentgreater than 5, wherein the contactor has less than about 20% of itsopen pore volume in pores with diameters greater than about 20 angstromsand less than about 1 micron, and wherein at least a portion of saidtarget gas component is adsorbed into said adsorbent material, therebyresulting in a product stream depleted of said target gas component; b)collecting said the product stream; c) desorbing the adsorbed gases fromsaid adsorbent material, thereby resulting in a waste gas stream rich insaid target gas component; and d) collecting said waste gas stream. 2.The process of claim 1 wherein the gas mixture is a syngas containingCO, H₂, and at least one other gas component selected from the groupconsisting of CO₂, H₂S, CH₄, and N₂.
 3. The process of claim 2 whereinthe target gas is selected from the group consisting of CO₂, H₂S, andN₂.
 4. The process of claim 1 wherein the adsorbent material iscomprised of a structured adsorbent material selected from the groupconsisting of zeolites, titanosilicates, ferrosilicates,stannosilicates, aluminophosphate molecular sieves (AlPOs), andsilicoaluminophosphate molecular sieves (SAPOs) and carbon molecularsieves.
 5. The process of claim 1 wherein the adsorbent material iscomprised of an 8-ring zeolite that has a Si to Al ratio of about 1:1 toabout 1000:1.
 6. The process of claim 5 wherein the 8-ring zeolite isDDR.
 7. The process of claim 5 wherein the 8-ring zeolite is selectedfrom Sigma-1 and ZSM-58.
 8. The process of claim 1 wherein the adsorbentmaterial is comprised of a zeolite selected from the group consisting ofMFI, faujasite, MCM-41 and Beta.
 9. The process of claim 1 wherein theadsorbent contactor is comprised of a first adsorption zone comprising afirst adsorbent material which is in fluid contact with a secondadsorption zone comprising a second adsorbent material, wherein thecomposition of the first adsorbent material is different from thecomposition of a second adsorbent material.
 10. The process of claim 9wherein the first adsorbent material has a selectivity for the targetgas component of the gas mixture over the second gas component greaterthan 5; the second adsorbent material has a selectivity for a third gascomponent over the second gas component greater than 5; and the secondadsorbent material has a greater adsorption uptake for the third gascomponent than the first adsorbent material.
 11. The process of claim 10wherein the target gas component is CO₂, the second gas component is CH₄and the third gas component is H₂S.
 12. The process of claim 10 whereinthe target gas component is N₂, the second gas component is CH₄ and thethird gas component is H₂S.
 13. The process of claim 10 wherein theadsorbent material is comprised of a structured adsorbent materialselected from the group consisting of zeolites, titanosilicates,ferrosilicates, stannosilicates, aluminophosphate molecular sieves(AIPOs), and silicoaluminophosphate molecular sieves (SAPOs) and carbonmolecular sieves.
 14. The process of claim 10 wherein the adsorbentmaterial is comprised of an 8-ring zeolite that has a Si to Al ratio ofabout 1:1 to about 1000:1.
 15. The process of claim 14 wherein the8-ring zeolite is DDR.
 16. The process of claim 14 wherein the 8-ringzeolite is selected from Sigma-1 and ZSM-58.
 17. The process of claim 10wherein the adsorbent material is comprised of a zeolite selected fromthe group consisting of MFI, faujasite, MCM-41 and Beta.
 18. The processof claim 1 wherein the adsorbent contactor has less than about 15% ofits open pore volume in pores with diameters greater than about 20angstroms and less than about 1 micron.
 19. The process of claim 1wherein the adsorbent contactor contains an effective amount of athermal mass material having a higher capacity for adsorbing heat thanthe adsorbent material.
 20. The process of claim 1 wherein the adsorbentcontactor contains both mesopores and macropores and wherein at leastsome of the mesopores and macropores are occupied with a blocking agentof an effective size that is small enough to fit into a mesopore but toolarge to fit into a micropore of the adsorbent material.
 21. The processof claim 20 wherein the blocking agent is selected from the groupconsisting of polymers, microporous materials, solid hydrocarbons, andliquids.
 22. The process of claim 1 wherein the adsorbent contactor is aparallel channel contactor.
 23. The process of claim 3 wherein theadsorbent contactor is a parallel channel contactor.
 24. The process ofclaim 13 wherein the adsorbent contactor is a parallel channelcontactor.
 25. The process of claim 22 wherein the adsorbent material iscomprised of DDR having a Si to Al ratio of about 1:1 to about 1000:1.26. The process of claim 24 wherein the adsorbent material is comprisedof DDR having a Si to Al ratio of about 1:1 to about 1000:1.
 27. Theprocess of claim 22 wherein the adsorbent material is comprised of an8-ring zeolite selected from Sigma-1 and ZSM-58 having a Si to Al ratioof about 1:1 to about 1000:1.
 28. The process of claim 23 wherein thetarget gas component is H₂S and the adsorbent material is comprised of astannosilicate.
 29. The process of claim 22 wherein the adsorbentcontactor is comprised of a first adsorption zone comprising a firstadsorbent material which is in fluid contact with a second adsorptionzone comprising a second adsorbent material, wherein the composition ofthe first adsorbent material is different from the composition of asecond adsorbent material.
 30. The process of claim 29 wherein the firstadsorbent material has a selectivity for the target gas component of thegas mixture over the second gas component greater than 5; the secondadsorbent material has a selectivity for a third gas component over thesecond gas component greater than 5; and the second adsorbent materialhas a greater adsorption uptake for the third gas component than thefirst adsorbent material.
 31. The process of claim 30 wherein the targetgas component is CO₂, the second gas component is CH₄ and the third gascomponent is H₂S.
 32. The process of claim 30 wherein the target gascomponent is N₂, the second gas component is CH₄ and the third gascomponent is H₂S.
 33. The process of claim 30 wherein the adsorbentcontactor has less than about 15% of its open pore volume in pores withdiameters greater than about 20 angstroms and less than about 1 micron.34. The process of claim 33 wherein the adsorbent material is comprisedof a structured adsorbent material selected from the group consisting ofzeolites, titanosilicates, ferrosilicates, stannosilicates,aluminophosphate molecular sieves (AlPOs), and silicoaluminophosphatemolecular sieves (SAPOs) and carbon molecular sieves.
 35. The process ofclaim 34 wherein the parallel channel contactor is in the form selectedfrom: a) monolith comprised of a microporous adsorbent; b) a monolithformed from a non-adsorbent material but whose channels are lined with amicroporous adsorbent; c) an array of hollow fibers comprised of amicroporous adsorbent; and d) laminated sheets having an upper and lowerface both of which are comprised of a microporous adsorbent.
 36. Theprocess of claim 35 wherein the channel gap of the open flow channels isfrom about 5 to about 1000 microns.
 37. The process of claim 35 whereinthe ratio of adsorbent volume to open flow channel volume is from about0.5:1 to about 100:1.